The Grinnell Glacier Trail in Glacier National Park is by far one of the best hiking trails in the park. It's a 12.2 mile out-and-back hike that leads up to Grinnell Glacier - one of the last glaciers in the lower 48 US states. This day hike can be strenuous, especially at these elevations.
The trail brings you past 3 large lakes, which are formed from snow melt and glacier runoff from high above in the mountains. Just before you reach Grinnell Glacier, you'll see the beautiful waterfall glacial runoff waterfall that descends to the lakes below.
There are numerous lookout points and photo-ops along the trail and you may even see wildlife like mountain goats (including longhorns), moose, and even bear. It's recommended to bring bear mace and a bear bell just in case.