Not only is the cooler weather better for hiking, the crowds die down as well. Plus, the valley transforms into vibrant shades of red and orange - a stunning sight!
Average highs/lows for the fall: September: 84 F/ 50 F October: 72 F / 41 F November: 57 F / 32 F
The best month to visit Yosemite and see foliage is in October. Leaves usually haven't turned in September, and November is a gamble because there may be snow.
The best things to do in Yosemite to see fall foliage is to hike the Valley Loop Trail, and the Sentinel Meadow/Cook's Meadow Loop.
Park reservations are only required through September 30 if visiting between 6am and 4pm. If visiting in October or November, they are not necessary.