The Smithsonian has two Air and Space Museums in the DC area. We took the boys to the one downtown off the National Mall. They enjoyed the Metro bus ride from the airport which dropped us off almost at the museum steps. And both boys (aged 4 and almost 2) had a blast at the museum proper.
There were tons of things to do. The airplane model that they could fly sparked toddler tantrums. Spencer, 23 months, was like Fred and Barney from the Flintstones. His little feet would start working before we put him down. Delighted, he would dart from here to there. The space shuttle replica and the rovers were his favorites. He thoroughly enjoyed the interactive room that had lots of experiences (as Edmund calls them) to try.
Edmund was thrilled to find that he could read some of the placards on the exhibits. He and bunny nearly danced a jig over the Wright brothers room. There was a life sized replica of the glider complete with navigation sticks that children of all ages could try. He had studied lift and thrust at school. So, to find them at the museum was fascinating. The astronomy section sparked a little kid commentary about the difference between asteroids and comets. We made a module to go on a virtual international space station. Robert took him to see the flight simulators. He enjoyed the ones at WonderWorks, but they had nothing on the ones at the Smithsonian!
Neither child was old enough to enjoy the National Mall. But the hustle and bustle of the capital seemed to interest them. Both children wanted to ride the train. Both children enjoyed watching the airplanes at the airport. And both boys loved the Air and Space Museum.