The Udvar-Hazy Center is the annex of the National Air and Space Museum on the Mall, and it exists because the Mall building simply couldn't hold everything. The result is a pair of vast aviation hangars near Dulles Airport where full-sized aircraft are displayed in stacked rows — nose to tail, wing over wing — giving the whole place the feel of a very tidy boneyard. The centerpiece is Space Shuttle Discovery, retired in 2012 after 39 missions, displayed alongside an SR-71 Blackbird, a Lockheed U-2, a Boeing B-29 Enola Gay (the aircraft that dropped the first atomic bomb on Hiroshima), and an Air France Concorde. There is also an IMAX theater and an observation tower overlooking the active runways at Dulles.
Admission to the center itself is free, as with all Smithsonian museums, though parking costs $20. Plan two to three hours if you want to read the placards; the sheer density of aircraft can be overwhelming. The hangar layout means sightlines are often blocked until you round a corner and something enormous appears at eye level. Children tend to do well here — the scale makes abstract history concrete in a way that captions alone cannot.