The U.S. Space and Rocket Center is the official NASA visitor center for the Marshall Space Flight Center (where Kristin works)! General Admission tickets are $30 and there are some discounts available. Check their website for details.
Saturday and Sunday are by far the best days to visit MidCity. There are several neat local shops and restaurants along with some more recognizable ones. There are also live events hosted at MidCity, so check their calendar for when you'll be in town to see what's happening!
If you're looking for hiking and other outdoor activities, Rainbow Mountain Nature Preserve is the closest set of trails in the Huntsville area. There are trails for all levels of difficulty.