None of these are obligatory, they just exist to get more out of the game.
Participate at your own discretion!

Wait for TORIEL
In the RUINS, if you wait for Toriel like a good child, after the first 5-7 minutes you will receive a phone call. There's about 7 more over the next 30 minutes. This isn't important, just for fun! You'll know when you've got the last call when it's something like: Snore... snore... *click*

Spider Bake Sale
If you buy a Spider Donut or Spider Cider here, they can be used later to end the fight against Muffet early. But why would you want to do that? Ahuhuhuhu~

Piano Puzzle
Correct Notes: ○ ↑ → ○ ↓ ↓ →
The treasure is a red herring, what you're truly after is the Dog Residue left in your inventory by the Annoying Dog. Using it randomly creatures more residue or "Dog Salad" - a useful RNG based healing item.

Temmie Village
Two screens right of the "Waterfall" boat location + turtle shop.
You can walk the path regardless of whether or not it's lit up, but it will be lit up after you touch the final mushroom on the way to the next room.
If you send Tem to college, she'll come back and stock a piece of armour at 9999g. The price lowers based on how many Game Overs you have.

Room Service
1 Buy hot dogs from sans in Hotland.
2 ... Keep buying hot dogs (and cats???), just for fun.
3 Take a hot dog to the last room in the right-side hall in the hotel.
4 Slide hot dog under the door. Somehow.
5 Receive Hush Puppy!

Mystery Key
Outside the hotel, you can visit Catty & Bratty's shop in the alley to the right. There you can purchase the "Mystery Key" for 600g.
The key can be used to open the house next to Napstablook's in waterfall.

sans.
Ever wondered how to get into his bedroom? ; )
When you reach the orange-hued, cathedral-like room where sans bestows judgement upon you, hear what he has to say then quit and reload your save - make sure you saved BEFORE talking to him. Repeat this 3 times and he'll give you the key to his bedroom. You fartmaster.
In there, you'll find another key, which opens the secret door behind their house.

Secret Door
To open the mysterious door outside of Snowdin, you need to successfully dodge everything in the mini-game portion of the True End credits. As of yet, there's nothing but a doggy dev room back there!

An Old Friend
After the True End, you're able to walk all the way to the beginning of the RUINS, where you'll find and be able to talk to Asriel in his true form. It is currently unknown - or not in the game yet - if there is any way to save him. ;_;

During the first time you meet Sans, press J during the cutscene to zoom in on Sans and crash the game. (Or your game could just completely glitch out.)




The reflections in the puddles of Waterfall can be changed if you press Spacebar, to be a old version of frisk.