Surely you haven't visited all the Smithsonian. If nothing else, visit the Science & Technology museum.
The Tomb of the Unknown Soldier at Arlington is well worth a visit. The discipline, especially in this weather, is incredible. If you can catch the Changing of the Guards it will be a treat.
What other things are you interested in generally? What mode of transportation do you have?

(im off to see that before going home). Also have a look out for some of the bits out of die hard 4 in Washington (i went looking for a few bits out of die hard 3 here in New York...)Visit Megaton.
) fallout 3.Take a diet plan with you.
lol'd