The Mississippi Museum of Natural Science is fun for both children and adults. The price is very reasonable. Parking at the museum is only $3.00 and allows you access to the park that surrounds the museum area. It's a good idea to bring a picnic lunch. There is also a hiking trail that is especially beautiful in the fall.
If you visit during the week, there are usually few visitors. This is especially nice if you wish to sit for a while and watch the live fish, turtles, and alligators. It is quite tranquil.