Air and Space Museum, Dulles, May 31
Any aviation enthusiasts with a longish stopover in DC should check out the National Air and Space Museum's Steven F. Udvar-Hazy Center, a short bus or taxi ride from the airport. It's basically a giant hangar full of wonderful flying machines - anything that was too big for the Air and Space Museum on the Mall. The star of the show is the shuttle Enterprise:
I'd also like to recommend a book I picked up there, Into That Silent Sea: Trailblazers of the Space Era, 1961-1965, by Francis French and Colin Burgess. It is full of anecdotes in context from the people who made early space exploration happen in both the US and Russia. Some of the stories contradict each other, but that's part of the appeal.