but if you want a more lively evening atmosphere but don't want to be too far away, check out the Georgetown area in the Foggy Bottom district, or possibly even down in Alexandria.
As for top things to do and see, here are my favorites in relative order:
Museum of Natural History
Air and Space Museum
Washington Monument
Lincoln Memorial
Jefferson Memorial
Tour of the Capitol
Vietnam memorial and WWII memorial
National Zoo (especially the orangutan exhibits)
Sculpture garden
Ford Theater
Museum of Art
National Archives

I wasn't impressed with the Native American museum. Compared to the one in Cody, Wyoming it felt really lacking. I didn't go to the holocaust museum because I've seen so much of that stuff from my bioethics course at BYU and other history classes, that I don't want to see it anymore. I understand what happened and see it as a complete tragedy that I hope we learn from, but I have no desire to revisit those sick feelings over and over again.