We recently visited the Nasa Goddard Flight Center Visitor Center. It has been on our list of places we have to visit, and this fall, we made it happen. It is a fun place. My boys aren't quite old enough or patient enough to sit and read and learn at these kinds of places, they would rather just do. The visitor center offered alot to do. They had many interactive displays, a few toys and an outdoor area that kids can run around and look at some old missiles and space structures.
The best thing about this place is that is that it is free! Here is the link for the visitor center
This is the kind of place that could a wide range of kids, it kept my kids entertained for a good hour and a half. I think older kids that could read and have an interest in space, they could spend alot of time here reading and interacting with the displays!