The John C. Stennis Space Center is the largest NASA-run rocket testing area in the United States and it's located on the border of Louisiana and Mississippi. The testing grounds are still in use, but they've recently opened the INFINITY Science Center so the public can come and learn more about the space program, rockets, and have a look at the facilities.
One of the coolest parts of the visit is the Omega Flight Simulator, a massive, moving simulator with a huge HD screen that makes you feel like you'r rocketing through the atmosphere. Apparently, one astronaut said that it was a fairly true representation of what it feels like to ride a spaceship. If you're looking for something a bit more educational, you have the Science Express, an area that details the history of moderns scientific exploration and the NASA Space Program.