U.S. News Insider Tip: Wander 20 minutes north to reach Hospital de Sant Pau. This elaborate, UNESCO-listed former hospital was the brainchild of architect Lluis Domènech i Montaner and is a beautiful example of Catalan art nouveau architecture. – Laura French

From 1883 up until his death in 1926, Catalan art nouveau master Antoni Gaudí devoted himself to the construction of La Sagrada Família, a towering, Gothic-style-with-a-twist church. But even then, he was unable to finish it; Gaudí was known for saying "My client (God) is not in a hurry."

The Barri Gòtic, or Gothic Quarter, sits at the heart of the Ciutat Vella – the oldest part of Barcelona – and, considering its location next to the city center, is one of its liveliest neighborhoods. Here you'll find beautiful examples of Roman and Medieval-era architecture rubbing elbows with the many shops, restaurants, alfresco cafes, bars and clubs that line its narrow roads and picturesque plazas – and there are so many plazas to explore. Aside from Plaça de la Seu, which you'll no doubt end up in if you visit the Barcelona Cathedral, make sure you stop in the smaller Plaça Sant Felip Neri, which was bombed by Spanish dictator Francisco Franco during the Spanish Civil War (you can see scars from the attack on the church in the square). The palm tree-clad Plaça Reial is meanwhile much more energetic and usually buzzes until the wee hours of the morning. Another notable plaza is Plaça Sant Jaume, where the Catalan seat of government has been since the Middle Ages.

No matter where you end up in the Gothic Quarter, travelers say its Spanish splendor will leave you charmed long after you leave. Many enjoyed strolling its narrow alleys and admiring its atmospheric, back-in-time architecture, and several said it was the highlight of their stay. Some suggested taking part in a walking tour if you're interested in learning more about the history behind the neighborhood. 

U.S. News Insider Tip: From spring through fall, Casa Batlló hosts "Magic Nights" – a series of open-air concerts on the dragon roof terrace, with romantic views of the city. Tickets start at 59 euros (around $62) and include entry to Casa Batlló and a drink. – Laura French

The details highlighted in Casa Batlló show famous Catalan architect Antoni Gaudí at his best. Of all the Gaudí apartments in Barcelona, this is probably the most recognized (it's also a UNESCO World Heritage Site). Sitting down the street from Casa Milà, Casa Batlló is known for its vibrant colors, intricate tile work and skeletal terraces. The unconventional façade is inspired by the legend of St. George, the patron saint of Catalonia, who is said to have slayed a dragon to save the king’s daughter. The roof depicts the dragon's scaly back, while the skeletal balconies and boney windows are said to represent the dragon's previous victims (the legend goes that someone would be sacrificed every day so the dragon wouldn't take the whole town).

U.S. News Insider Tip: For more sweeping city views and fewer tourists, grab a five-minute taxi or walk 20 minutes uphill to reach Bunkers del Carmel – a viewpoint popular with locals, especially at sunset, when the lighting over the city is spectacular. – Laura French

Antoni Gaudí's Park Güell is as whimsical as parks can get. The park was originally supposed to be a housing community for the rich, commissioned by Eusebi Güell. Güell hired Gaudí but the project eventually folded due to the land's incompatible building conditions. Gaudí continued on, modeling the park after gardens he had seen in England (Güell means English in Catalan) and building around the natural elements of the land instead of tearing them down.

U.S. News Insider Tip: Keep your hands on your bags and keep an eye out for pickpockets on this notoriously busy thoroughfare. – Erin Evans, Managing Editor

This wide, tree-lined boulevard is one of the city's major tourist hubs – so much so that if you're visiting Barcelona, you're bound to end up here eventually. Las Ramblas is a pedestrian-friendly pathway situated right smack dab in the middle of the city, so expect it to be busy all hours of the day and night. During the day, you can peruse souvenir stands, watch buskers and street performers, pick up some local art from artists selling on the street, or sit down and enjoy a light snack at one of the many alfresco cafes found here. When the sun sets, head here to start your night out – many bars and clubs can be found in the surrounding area.

U.S. News Insider Tip: While Barceloneta is a must-see with its lively bars and restaurants, it can get crowded, so if you're looking for a quieter alternative, wander 20 minutes up the coast to Playa de Bogatell (it attracts more locals and fewer tourists). – Laura French

In between all the cultural and artistic attractions Barcelona has up its sleeves, it's easy to forget that the city is situated right along the brilliantly blue waters of the Mediterranean. The city's swathes of sand are broken up into several different beaches – among them Barceloneta and the quieter Nova Icària. Both are separated by the Port Olímpic harbor, easily recognized by the two seafront skyscrapers and giant golden fish sculpture by artist Frank Gehry.

The nickname, La Pedrera (meaning "the Quarry"), is appropriate for Antoni Gaudí's stately, fortress-like Casa Milà. Bobbing around the corner of Passeig de Gràcia and Carrer de Provença, this eclectic Catalan-style art nouveau building rubs elbows with the more classic architecture usually found in its neighborhood, Eixample. It is known for its wavy stone façades and intricate carvings that can only be attributed to Gaudí's quirky style. Casa Milà was originally constructed as a home for the commissioners of the building (Pere Milà i Camps and his wife), who also requested the complex included apartments for rent. Casa Milà was not only Gaudí's last work on Passeig de Gracia (Casa Batlló is just a few blocks south), but his last civil work as well. Since then, Casa Milà has been designated as a National Monument of Interest by the Spanish government and a World Heritage Site by UNESCO.

Today, La Pedrera is a cultural center managed by the Catalunya La Pedrera Foundation. Come here to admire the architecture: there are few single straight walls or right-angled corners in the entire building. Also head to the roof to get a good look at Gaudí's whimsical chimneys (many designed to look like guardian warriors in armor), then work your way down through the exhibits to learn more about Casa Milà and Gaudí himself.

Even if you're not keen on visiting the touristy Las Ramblas, it's worth making the trek to this tree-lined thoroughfare to reach the foodie heaven that is the Boqueria Market. This was Barcelona's oldest local market, having opened in 1840 – but its foodie history spans much earlier than that. The first food vendors were said to have been around as early as the 13th century, selling meat on the streets. The market you see today wasn't around back then; it took four years to construct, once Saint Joseph's convent left the area (hence the name of the market). 

Today, that tradition of hawking goodies lives on, and the covered marketplace treats visitors to the vibrant colors and enticing aromas of everything from fruit juices and wines to fresh fish, meats, produce and desserts. Make sure to grab Spanish specialties while you're there, including jamón ibérico, manchego cheese and salted cod (or bacalao). Bars and restaurants can also be found in and around the market, so food options truly abound here. 

Barcelona's Palau de la Música Catalana is considered to be a masterpiece of Catalan art nouveau. Built by architect Lluís Domènech i Montaner, the palace earned the title of a UNESCO World Heritage Site for its striking architectural features. Outside, make sure to snap a few photos of the intricate mosaic pillars and the busts nestled atop some of them, which depict famous musicians, such as Bach and Beethoven. The interior of the palace is even more of an eyeful, complete with mosaic pillars and intricate sculpture work of its own, as well as stained glass windows and beautiful motifs of flowers spread throughout. And you won't be able to miss the massive stained-glass central skylight – it protrudes from the ceiling, treating the concert auditorium to plenty of natural light. Aesthetics aside, the Palace of Catalan Music is a hub for classical and choral music and of course, Catalan musical arts. It also acts as a concert venue for local, national and international acts.

Travelers agree with the experts: the Palau de la Musica Catalana is an architectural marvel. Visitors thoroughly enjoyed admiring the many intricate details found throughout the music venue, saying even if you can't get tickets to a show, it's worth a visit just to see its magnificence in person. Some weren't crazy about the admission price, but many enjoyed the guided tour, and those who did attend a show said the experience was quite magical. 

U.S. News Insider Tip: You can visit for free on Thursday evenings (from 5 to 7 p.m.), all day on the first Sunday of the month and on special Open Door days; check here for details. – Laura French

When you feel like you've hit your Gaudí limit, head to the Picasso Museum (Museu Picasso) for a change of pace. While most people know Pablo Picasso for his distorted portraits, this museum displays his work on a timeline of sorts, allowing you to follow his progression from the more controlled works of his early years to the very whimsical paintings and sculptures from the end of his career. Make sure you dedicate plenty of time to Picasso: the museum itself holds around 5,000 pieces by him, including works from his famous Blue Period. The museum also explores the artist's lifelong relationship with Barcelona, explaining why he chose the city for his museum before he died. 

If you have even the slightest interest in history, make sure to add Montjuïc Castle to your Barcelona itinerary – the stone structure is teeming with history dating all the way back to the 11th century. The castle started out as a single watchtower that was occupied by a sailor looking out for enemy ships. During the Revolt of Catalonia during the mid-1600s, the government decided to add walls surrounding the watch tower when the threat of invasion from Spanish King Philip IV's fleet became imminent. Montjuïc Castle ended up defending the city from many attacks moving forward, including those carried out during the War of the Spanish Succession. It also served as a prison during the War of the Pyrenees and was occupied by Napoleon's troops in the early 1800s.

Montjuïc continued to serve as a prison under multiple political leaderships over the course of the 18th and 19th centuries, including during the Spanish Civil War. Francisco Franco, Spain's dictator from 1939 to 1975, took over Montjuïc and it became an internment camp for Republican soldiers (Franco was part of the Nationalist party that overthrew the democratic Republic of Spain at the time). It was here that the President of the Catalan Government, Lluís Companys, was executed at Franco's orders. 

Towering above the center of the Barri Gòtic district is Barcelona's principal cathedral. The Gothic cathedral's construction began in the late 13th century, though it wasn't completed until the mid-15th century. While you're here, take time to explore the numerous examples of artisanship that went into completing the cathedral – from its exterior details to the many gold furnishings within, including its elaborate baroque altarpieces, as well as the 140-plus statues of saints that call the cathedral home. While you're here, make sure to stroll over to the cloister, which features a verdant tropical garden. 

Recent travelers found the Catedral de Barcelona to be stunning both inside and out, commenting on the elaborate altarpieces, stained-glass windows and impressive statues. Many recommended a stroll through the cloisters to see its pond, home to several geese, and climbing to the roof of the cathedral to get an eyeful of the spire up close, as well as prime city views. Make sure to wear the proper attire; according to recent visitors, the dress code here is strict and knees and shoulders should be covered. Shawls are said to be for sale for those who need to cover up. 

U.S. News Insider Tip: Come in late August for the free Festa Major de Gràcia (Gràcia Festival). During the weeklong celebration, streets come alive with colorful lanterns and other creations, competing to be the best decorated; expect live music, food, parades and Catalan correfoc fire displays. – Laura French

If you've already seen the bucket list sites and want to escape the crowds, amble north of the city to explore the charming, bohemian streets of Gràcia. Once its own separate town, this elegant, colorful neighborhood has kept its independent village feel, with cobbled streets, lively plazas and laid-back cafes aplenty. At its heart is Calle Verdi, lined with trendy boutiques, vintage stores, bookshops and restaurants, and Plaça de la Virreina, where locals gather on outdoor cafe terraces in the shadows of the Church of Saint Joan. While you're here, pay a visit to the UNESCO-listed Casa Vicens – Gaudí's first home in Barcelona, now a museum – and the Mercat de la Llibertat, a large food market where stalls overflow with fresh produce between brick-and-iron architecture.

U.S. News Insider Tip: A five-minute walk from the park will take you to El Born – one of the city's trendiest neighborhoods, with tapas bars, indie boutiques and bistros lining cobbled lanes. Head to the Bormuth tapas bar for great food in a lively atmosphere. – Laura French

Built in the late 19th century on the grounds of a former citadel, Parc de la Ciutadella is one of Barcelona's most popular green spaces. At its heart you'll find an elaborate fountain, surrounded by sculptures and designed by architect Josep Fontserè i Mestre – who asked Gaudí, then still at university, to help out (visitors might recognize Gaudí's iconic style in the winged dragons guarding the fountain).

The highest mountain in the Serra de Collserola range, pine-forested Mount Tibidabo peers over the city, crowned by its neo-Gothic basilica, the Temple Expiatori del Sagrat Cor (or Temple of the Sacred Heart of Jesus). Built in the early 20th century in the northwest of the city, this elaborate, Catalan art nouveau-style masterpiece can be seen from almost anywhere in Barcelona and is well worth a visit in itself – but it's the views over the city that really astound here. A lift and several steps take you up to the tower, where a viewing platform offers a full panorama of the skyline and sea below.

It's not just the church that lures visitors to the mountain, though; it's also home to Tibidabo Amusement Park, one of the oldest theme parks in the world and the oldest in Spain, with a carrousel, bumper cars, Ferris wheel and several other attractions drawing families. Hiking trails are on the doorstep too, with much of the area protected under the Parc Natural de Collserola – the largest green space in Barcelona and the city's green lung.

There are ample reasons to visit the impressive Boqueria Market, but if you're after a less-touristy alternative, put Mercat de Sant Antoni on your list too. Located in the up-and-coming Sant Antoni neighborhood, the market originally opened in 1882 in an art nouveau, wrought-iron and glass building; neglected, it closed in 2009, but reopened nine years later following an 80 million euro refurbishment. Today, the market is home to a whole range of fresh products, from Spanish cured meats to fresh cheeses, fruit and vegetables, which sit beneath its striking, original octagonal domed ceiling. You'll also find stalls selling clothes, shoes, homeware and more. And on Sundays, the surrounding streets (which have been semi-pedestrianized under one of the city's traffic-free "superblocks") turn into one of Europe's largest outdoor book fairs.

Beyond the market, there's plenty to explore in the area, too; its opening helped revamp the neighborhood, turning it into something of a foodie paradise with tapas bars, bodegas and restaurants on almost every corner.

Whether you're coming with kids or just want to embrace your inner child, Barcelona's science museum is well worth a visit. Housed in a modernist glass-and-steel building, it's home to an array of interactive exhibits ranging from the "Flooded Forest" – an Amazonian-style glasshouse home to piranhas, crocodiles and exotic plant species – to "The Universe Gallery," which focuses on all things space, from the creation of the cosmos to evolution and the human brain. Families can get hands-on with different experiments, from creating sandstorms and tornadoes to learning how soundwaves are transmitted, and there are some impressive design features too – don't miss the Foucault pendulum, a heavy iron ball used to demonstrate the earth's rotation; and the spiral staircase, which winds around a towering Amazonian Acariquara tree.

Recent visitors were highly impressed by the CosmoCaixa and enjoyed the interactive exhibits, especially the Amazonian rainforest section. Several said it was the best science museum they'd been to, with state-of-the-art facilities and plenty to see, and many recommended spending a few hours here. Others were surprised at how reasonably priced it was. A few said there wasn't much for very young children, but for many it provided a welcome escape from the heat and an impressive alternative to the better-known Barcelona sites.