Truth be told, all hikes in and around Mammoth Lakes are awesome in the Fall. The lower daytime temps in October and November are great for day hiking, the sun still shines pretty much everyday, and the trails are less crowded than peak season. It can get cold and even snow this time of year, so check the weather, but if its sunny and 70; and you want to see some fall colors; then here are a few great options.
Heart Lake trail is a pretty quick and easy hike for the average hiker. On of the great things about the trail is the payoff comes quick. The views from the trail are amazing as you hike up to Heart Lake. The hike is just over a mile and is accessed from the far end of the Coldwater Campground parking lot. There is a great description of the hike here.
Sherwin Lakes Trail is a medium difficulty hike just outside of town. It is a 2.5 mile trail (5 Miles up and back) accessible via Sherwin Creek Rd. You won't see to many fall colors until you get up to the lake, but lake is a great place to relax and take in an amazing view even on a cool day. More info on the hike here.
For the third and hardest hike, we will take you to June Lake. The Fern Lake Trail is not a very long hike at 3.4 miles total (out an back), but the elevation gain is an eyepopping 1,582 feet. The photo in this blog post is not from the Fall, but if you do this hike in October of November you will enjoy some of the best fall colors anywhere in the Eastern Sierra. The amazing views to the west on the ascent to Fern Lake start only about 10 minutes into the hike. The trailhead is located on the left side of the June Lake Loop after you pass the ski resort. Click here for a good description of the hike from Mono County Tourism.