The Smithsonian National Air and Space Museum in Washington DC is close to the US Capitol building and the National Gallery of Art. It's walking distance to all the other Smithsonians and the National Mall. This is one of my kids' favorite museums, and they look forward to visiting, probably because of the total geek factor of the exhibits, and lot's on walk-through exhibits and hands-on stations.
Hands on exhibit "How things fly"
First private space flight vehicle.
Mars Viking Lander
First round-the-world air balloon flight
First private space flight vehicle
Apollo Command Module
So much fun, and *free* to visit! (You know I love that!) For more on the Smithsonian National Air and Space Museum, click here.