I've heard great things about the Museum of the Rockies, but not been there myself yet. I'm not sure your average 15 yr old would enjoy it, but I could be wrong.
The Elk Refuge sleigh rides are great...
Also, the snowmobile/dogsledding companies go to Granite Hot Springs south of Jackson... I plan to do that this winter. I enjoyed the snowmobile to Old Faithful a few yrs back, but it was a blizzard all day, so although I wasn't cold, we didn't see too much.