You can try a different outdoor adventure every day of the week in the Flathead Valley.  Glacier National Park is breathtaking and an alpine hike is a must during your visit.  The mountain bike trails down Big Mountain provide a thrilling ride, and you can let the lift do the hard part.  Take a raft trip down the Middle Fork of the Flathead, one of the few rivers in Montana given the designation wild and scenic.  Fly-fish for rainbow trout in a pristine stream.  Golf a few rounds at one of the many courses nearby, or just pick up an inner tube and take a lazy float down the Stillwater River.