I have heard nothing but great things about the Aquarium! Tickets are $40 for adults and $30 for ages (5-17). Children under 4 are free!
If you get a chance to explore, Lookout Mountain has some stunning views! An easy way to get to the top is by taking the Incline Railway. It's an easy ride and wheelchair permitted, and the tickets are reasonably priced! ($15)
In addition to Lookout Mountain there is an attraction located nearby that has an underground waterfall! Tickets are $25 for adults and $15 for children.