The Mormon Pioneer Trail near Salt Lake City, Utah, climbs from near Little Dell Reservoir up to Big Mountain Pass, and it follows the Donner Party and Mormon pioneers. This trail is fairly smooth, but the climb through the switchbacks make you earn the excellent descent. It’s easy-intermediate technical, but pretty strenuous aerobic exercise.
While the start of the trail can be warm, the actual climb is definitely cooler, as the trees raise the humidity and block the breeze. It will actually feel cooler when you reach the exposed hillside and start climbing in earnest.
As this is an out and back, be sure to watch for riders coming up and down the trail. There may be a few hikers as well, but this is largely a mountain bike ride.