The one thing our 16-year-old wanted to do on our trip to Yellowstone was go white water rafting. There's a few options in Gardiner, MT but we went with Wild West Whitewater Rafting on a recommendation from an employee at the motel we were staying at and we're so glad we did! They're located right in the middle of town and everyone there was helpful and kind. They have all the equipment you need (water shoes, splash jackets, etc). You can also pay for a picture package, which we got. It was worth the cost! There's no way you can get those great pictures yourself when you're on the boat.
Matt was our guide, he was great and very knowledgeable about the area and Yellowstone. We only did the 2 hour trip, but wished we had done the full day, especially our 16-year-old.