Yellowstone offers 200 miles of snowmobile trails. Although the noisy machines scare away the wildlife you might see en route, most tours stop at Old Faithful, a gathering place for bison drawn to the warmth of the geysers. When my children were teens, we rode snowmobiles for hours, all the way from Jackson Hole, W.Y., to Old Faithful and back again. I returned exhausted and cold. My teens returned exhausted but exhilarated. Some outfitters allow two-people on a snowmobile. In general, we recommend shorter treks than our all-day outing and then only for driving age and older teens that can follow directions and resist racing. Be sure to pick a reliable company. Check the park’s Web site for permitted outfitters.