I sorta live in the area. I am currently in Middle TN, about 1.5 hr from Chattanooga, and I grew up in east TN, so I have been to Gatlinburg and Pigeon Forge more times than I could ever remember.
The aquarium in Chattanooga is MUCH better than Gatlinburg. What makes it cool is that it not only has a salt water aquaruim, it also has a very large fresh water display, which you don't see all that often. In fact the aquarium was only fresh water for several years. They also added a penguin exhibit recently. I haven't gotten a chance to see it yet though.
On your cades cove day, stop by the Smokey Mt. Visitor's center - which is cool in and of itself, and get directions to Laurel Falls. It is an easy hike, and the trail is paved. The water fall is beautiful.
You might want to visit Ober Gatlinburg. You take an aireal tram up the mountain or you can drive. They have an indoor ice skating rink, some intersting shopping and a black bear exhibit.
I really couldn't think of anything specifically for DD's birthday, but I think she will enjoy Biltmore. It was one of my favorite places as a kid.
Feel free to PM me if you have any specific questions.
