Huntsville is very cliquish IMO. Finding out about something of interest is tough if you are not already in the know. In general, there is a over-hyped feeling of all things SPACE with a hefty side of Old South nostagia. That said, over the last however many years, the options have expanded a bit beyond the Space & Rocket Center. The Convention & Visitors’ Bureau was able to convince a travel blogger organization to host their convention in Huntsville this coming weekend - go figure.
The arts scene isn’t huge, but fairly active (http://ourvalleyevents.com/, http://www.lowemill.net/). The downtown area used to be the private domain of law offices and bail bondsmen but has been reanimated with some decent, local, non-chain restaurants (even Paleo, gluten-sensitive, veggie options), several craft breweries and periodic public events (http://www.downtownhuntsville.org/). The Huntsville Land Trust (http://www.landtrustnal.org/) maintains a system of hiking and biking trails and offers hikes further afield. I hear occasional rumors of attempts at a music scene, but I am skeptical.
If you have a reason to be in town, there are things to do, the options are more varied than they were even 5 years ago and you’ll probably have an enjoyable visit. But if you’re looking for a standout, bucket list type reason to come to Huntsville, nothing really comes to mind.