The Noog
Dunno if you're still in town, but if you have kids, take them to the Creative Discovery Museum (or if you're a kid at heart, you'll probably like it too). The IMAX is nothing special compared to others. If you're a history buff, you might like some of the Civil War sights (Craven's house and Point Park, Chickamauga Battlefield, Battles for Chattanooga Museum, et al.). If you're the artsy type, try the Bluffview Art District (a couple of good restaurants, a yuppie coffee shop, a statue garden, and Hunter and Houston art museums).
For nightlife, Chattanooga Billiards Club is a decent place to shoot pool or have a few drinks if you want a nice-ish place, or Parkway Billiards for a seedier touch with better company (my personal favorite place to unwind). The Terminal is a microbrewery with pretty good food (non-smoking), or Hair of the Dog for it's smoking equivalent. Big River is a decent restaurant/microbrewery (try the Vienna Lager, you'll thank me) which is attached to Rhythm and Brews--a bar where you might be able to catch a decent show. Easy Bistro is one of the only Chef owned/operated restaurants downtown. It's upscale (still no dress code, I can personally verify that they'll let you in in Carhardts and a t-shirt) and the food is absolutely incredible.
If you need more, let me know. I've made just about every mistake there is to be made here over the last 20 years.
PS: if you manage to hurt yourself today, Rock City is in my EMS district and I'm working today