Nightlife and the Monuments
I love the lights, the nightlife, the crowds. Even though DC is one of those cities whose bars and clubs close at like 3am, which can be considered early, it's still pretty cool. DC in my opinion doesn't exactly have the best nightlife out of many cities...but it's still pretty good.
Georgetown and Adams Morgan are the usual hotspots for many people here, including myself. Adams Morgan in my opinion is tons better though. The last place here that I went to is this bar called Ghana Cafe. They play good music and it's a really good place to dance.
Another cool thing that I recommend to everyone is to go see the monuments at night. They are absolutely beautiful on a nice spring or summer night, and there's less of a crowd too. My favorites are the World War II Memorial and the FDR Memorial.