It all depends on what you want and what interests you. I've heard the spy museum (
http://www.spymuseum.org/) is supposed to be really good. I haven't been there yet but next time I'm in D.C., I will definitely check that out. Maybe when I pass through there on my way to N.C. for Thanksgiving. There's also the National Archives if seeing the original documents that built this country interests you. The Smithsonian is always a great place to go. Personally, I'd skip the White House tour. It was freezing cold when I went, stood on a long line, and just wasn't all that impressed with what I saw. The Washington Monument has a nice view from the top. The Steven F. Udvar-Hazy Center (
http://www.nasm.si.edu/udvarhazy/) located outside of D.C. (near Dulles) is pretty good. Lots of aircraft inside including an SR-71 Blackbird, Concorde, and a Space Shuttle (although it is not one that went into space, it is actual size and quite impressive to stand next to). Some of the war memorials are also impressive to visit (Vietnam Memorial, WWII memorial, and Arlington National Cemetery, including the tomb of the unknown soldier). Here's another good webiste to find things to do down there (www.washington.org/).