Allow at least 4 hours. More is better.
There's a lot more than the Rocket Garden to see at Huntsville. There are exhibits inside that should not be missed. It would have been nice if we could have explored the simulators but they were reserved for the kids attending Space Camp. Something you can do is make use of the Apollo Command Module simulator. If you have seen movies of men inside this capsule, you may be surprized at how little room there really is. Something that caught my attention was a huge piece of equipment that contained the computers, sensors, and telemetry equipment that made up the 'brain' of the Saturn 5 rocket.
Home of NASA Preschool
I've been to Huntsville three times in my life. The first two times were to attend the Space Academy and Aviation Challenge week-long camps during middle and high school. But the third time was as a senior in college, where I was a part of the 2003 Great MoonBuggy Race competition.
The MoonBuggy Race took place at the Rocket Park, part of the Space Camp grounds. It was really cool to see the place again - I hadn't been there for about ten years! The racecourse was 0.7 miles long and took us on a winding path around various rockets and imitation obstacles - craters, gravel pits, ramps, etc. The Buggy had to be human powered, and our design had me facing backwards - which turned out to be a good thing!
Unfortunately, out of about 32 college teams we came in 17th. We could have done a LOT better but there was a mechanical problem - my bike chain wasn't tensioned correctly and it kept popping off the gears. So for the *entire* race I was pulling the wheels with my arms! If our design had had me facing forward it would have been a lot harder. I must say, pulling a 400-pound vehicle and cargo for 0.7 miles is a lot harder than it sounds! Fortunately Scott's chain held up and between the two of us we were able to finish the race. Nearly half of the contestants can't even say that. It was a tough course!