I've yet to go myself, but I hear the Holocaust museum is a must-see (bring a hanky, they say).

If you like pre-Columbian art (as I do), check out the Dumbarton Oaks museum in Georgetown.

Chinatown. Eat dim sum in Chinatown.

Ride the Metro. The stations' architecture and lighting is very calming. Check out the Wheaton Metro station -- it has the longest escalator in the Western Hemisphere. (Rosslyn and Dupont Circle are not far behind.)

tooki