This past Saturday I completed the Devil's Backbone 50 mile ultramarathon, which is a big out and back from Hyalite Creek on the Gallatin Crest. Hiking (I gave up running last year) 50 miles in a day is not what I want to discuss here.
What I want to talk about is the crest trail, which was totally brilliant. Nice, faintish trail, challenging walking, and otherworldy views in all directions. There isn't a ton of water up there, but it would be a primo desination for a day hike or backpack (and would have some reediculous spring skiing).
Here are some topo maps from the race site: http://www.math.montana.edu/~thayes/Runs/Devil/DevilMap.html
I put together a goofy little video of the race: http://vimeo.com/5571872
Them BPL people have a pretty nice backyard!