The S Bahn aka Trainride to the city is only 11 minutes, 15 minutes to the center Hauptwache, so easy to go out at night and stroll around the city. There are some nice restaurants around, especially at Fressgass. You did not mention what time of the year, Sachsenhausen is nice with there old, authentic Applewine Taverns, easy to be reached via the same S Bahn train from the Airport to Konstablerwache, follow the bus signs , bus line 30 and 36 takes you to the old town. It is a 5 min bus ride, exit Affentorplatz.. Well worth it. There you can enjoy local food like schnitzel and green souce. Dauth Schneider, Atschel, Struwwelpeter, Fichtekranzi are all by Affentorplatz and have excellent food