Aside from Tigger's great suggestions, may I add the following:
1. Huntsville Botanical Garden
2. Railroad Museum and (occasional) Steam Locomotive Train ride
3. Lake Guntersville (1 hour away) fishing and boating
4. Spelunking (sp?). Much of Huntsville sits atop a vast cave network.
5. SCUBA Diving (Madison Water Park)
6. Huntsville Symphony Orchestra (HSO)
7. Concerts in the Park (Spring - Fall) by HSO
8. Jack Daniels Distillery (just under 2 hours away in Nashville)
There are more if you want to hear them. :-D
Regards,
Ltstanfo