Both my sons (ages 7 and 10) loved the Nt'l Air and Space Museum. In fact, we all had a blast exploring the many exhibits throughout the museum including rockets, shuttles, and a replica of the Hubble Telescope. The boys enjoyed the wonderful history of the Wright Brothers and their many attempts to fly. Inside the museum is also an IMAX theater (although not free) it was definitely worth watching a show. There's a great gift shop and a cafe for those tired feet to relax (although it was a bit crowded on a Saturday).