go to the botanical gardens! they have just redone it and it is fun. Lots of cool plants! it is a few blocks up (towards the capital) from the air and space museum.
Natural history museum is fun, has the hope diamond, an insect zoo, some cool exhibit about earthquakes... has a map that is updated every 10 minutes with the latest earthquakes
air and space museum.... very cool. there is an entire section of hands on stuff for kids with some cool stuff. The Imax movies are here and at the natural history musuem. If "Too Fly" is still there (it was the first one I think) it's cool. (the dinosaur one over at the natural hx museum you can skip, it's pretty lame)
mount vernon is pretty nice, nice tour of George's house, the grounds are gorgeous! well worth a trip. It's not very far from dc, it's in alexandria va.
the tour-mobile seems pretty cool, I think it's roughly 18$/person but I think you can hop on and off all day at various places. Look up tour-mobile and wash. dc and it should give you info.
The national zoo rocks! it's hilly, but having just been out to the san diego zoo, it's a piece of cake. (and personally, I like our zoo better. You can see the pandas (if they are outside) from the patio overlooking their enclosure... it's almost all the way at the top of the zoo across from zebra and next to the american buffalo (no line). There are a bunch of "baby" animals... they are probably all 1-2 yrs old now, but stll cute.
Arlington cemetary and arlington house (general lee's home at one point I think, or his sister's....) See the house if you can, its pretty neat.
See if you can get tix to the white house, although I don't think they are doing tours anymore.... can't keep straight what is closed still to the public these days.
Try and get tix from your congress person to the capital.
The supreme court may be open....
Spy museum, national building museum, post office museum, museum of african art....
feel free to email me, I live in the dc area and would be happy to answer any questions. We will also be in town if you get stuck or need anything.

maggie