Air and Space, the Spy Museum and the Smithsonian in general. Avoid the WH as others have said, you don't see much of anything and the lines are horrendous. The Vietnam Memorial is a somber site but definitely worth visiting. I like the Jefferson and Lincoln memorials. Visit Tom Sarris' Orleans House at the Rosslyn Metro stop for some awesome prime rib.
Aw man. I've seen that place while I was driving down Wilson on my way to Georgetown. Now I'll have to stop in.