One of Yellowstone National Park’s concessionaires, Xanterra (307/344-7901, www.travelyellowstone.com [1]), runs bus tours of the park. From late May through mid-September, buses leave Gardiner [2] every morning at 7:45 a.m., tour the Grand Loop of Yellowstone’s roads, and return to Gardiner at 6:15 p.m. The fee is $65 adults, $32.50 kids 8–15, free for children under age 8.
Links:
[1] http://www.travelyellowstone.com
[2] http://www.moon.com/destinations/montana/the-missouri-headwaters-and-south-central-montana/livingston-and-the-paradise-valley/gardiner