Concierge.com's insider take:

The Gallatin River may be best known as a fly fisherman's dream, but punctuating all those fishing holes are some spectacular rapids (up to Class IV). Montana Whitewater's trips down the winding steeps of the Gallatin Canyon range from relatively mellow stretches upstream to the nonstop rapids of the aptly named and terrifying Mad Mile and the massive obstacle known as House Rock (800-799-4465; www.montanawhitewater.com). Both half- and full-day trips are available year-round; although the winter air temperatures are bitter cold, the same fissures in the earth's mantle that heat Yellowstone's Old Faithful keep the Gallatin River streaming all winter long. If you're bold enough to raft in January, you'll be rewarded with a view of the rimed canyon: Steam from the river freezes on the surrounding trees and cliffs to create a frosty wonderland beyond compare.