We really enjoyed Wahoo! Ziplines in Sevierville.
They bill themselves as the largest zipline in the US, and they might be right. Six long runs through scenic hills and valleys -- NOTHING like the touristy places along the main parkway through Sevierville. We were on the ziplines for well over an hour. Our guides were both Russian, and they were a hoot! Highly recommended.
We did the Little Pigeon River "whitewater" rafting trip. I forget which outfitter we used, but the whole thing was a yawn. When the guide has to steer the boat into a wave to get you slightly wet, you know you should have been tubing. Our outfitter was fine, but this is a very poor excuse for "whitewater."
The Cades Cove trip in Great Smokys National Park is a MUST DO. Try to arrive very early -- as soon as they open. It's a drive to get there from anywhere, so plan ahead and start early. If you go mid-morning, you will be crawling around the loop road at a snail's pace. We went early and took a Ranger-led hayride tour (at 7:30 AM, I think), which was excellent. Tours are seasonal, so check the Park website at
www.nps.gov/grsm a couple of weeks before your trip to get the current schedules.
Lots of other stuff to do in the national park. It's our most-visited national park, with more visitors every year than Yellowstone, Grand Canyon, and Yosemite
combined, and there is a LOT to do there if you value nature experiences. Check the link above, especially the link that says "Plan your visit."
We also made the trek across the mountains to Cherokee and attended "Unto These Hills." This is a very good outdoor drama about the Cherokee Indians and the "Trail of Tears." Reasonably priced, historically accurate, and well-done.
As others have said, the Sevierville/Pigeon Forge/Gatlingburg/Great Smokys NP area has tons to offer. You could spend several weeks there and not see everything.