Besides the Smithsonian museums and the monuments some of the things I have done with my family include:
FBI, White House and Capital (but you have to arrange in advance).
U.S. Mint and Holocaust Museum (right next to each other), National Geographic Museum
Pentagon and Arrlington National Cemetary.
And if you want to go a little out of town, the National Aquarium in Baltimore and Annapolis and the US Naval Academy.
My wife hates traveling with me because, I would try to see all of the above and possibly more, in a 3 or 4 day weekend.