-Eastern Market in DC is decent.
-Drive to Udvar-Hazy in Chantilly, VA.
-The FDR Memorial has impressed a few friends...I enjoy taking out-of-towners to the Jefferson Mem., or a drive out to Haines Point, down to Roosevelt Island.
-I think you can rent boats / bikes down along the canal in Georgetown.
(Is there still the flea market up Wisconsin Ave. in Georgetown? Or, is that once a month?)