The hike to the top of Avalanche Peak in Yellowstone National Park can be done in a morning (about 4 hours round-trip) and is absolutely worth the climb.
The access point for the trail (cleverly titled “Avalanche Peak Trail”) is about 8 miles inside the East Gate on the north side of the highway. There is a small parking lot and picnic area by Eleanor Lake, across the road from the trailhead.