National Air and Space Museum

The National Air and Space Museum is the most popular museum in the world, with more visiters than even the Louvre or Uffizi. It's also the largest collection of historic air- and spacecraft in the world. Where else can you see planes flown by the old greats like Lindbergh and Earhart, and newer pieces of aviation history like the Voyager plane (first non-stop, non-fueled flight around the world in 1986). And don't forget the space program, although in its case a lot of the craft you see on display are models- they don't always come back to Earth! The Mercury and Gemini programs, Apollo and Viking, up through today's space shuttles and the Hubble Space Telescope. NASM has two theaters; the Samuel P. Langley, which shows IMAX films, and the Albert Einstein Planetarium. Free (except for the theater shows). Metro: L'Enfant Plaza or Smithsonian

This is really a museum worth visiting!  The attractions are always changing so you can go over and over.