We flew down. The flight's about an hour, not bad. Spy Museum is cool, yes. So is the Nat'l Air and Space Museum. And the Nat'l Museum of Natural History (a bit...errm...creepy at night!).
If you go up to Mount Vernon (which we did) or Arlington Nat'l Cemetary (which we did...twice; once on Nat'l award trip, once on school trip) wear light clothing and BRING WATER!! WATER WATER WATER!!