North of Crested Butte, the ride up Oh Be Joyful creek to Blue Lake is a 6 mile one-way route. This is a popular route with both hikers and horseback riders.
The trailhead is at the Oh Be Joyful Campground on Slate River Road. Start by riding west up Oh Be Joyful towards Blue Lake. You will see a lot of other hikers on the way to the lake. Fantasy Ranch has guided horseback rides up this route.
About 0.25 miles before Blue Lake, there will be an small, unsigned trail meandering off one of the switchbacks heading east towards Star Pass (the low saddle in the ridge to the west). Its quite steep and loose, and not recommended for horses.
Overall, Oh Be Joyful is the 27th most popular horse trail of all 66 horseback rides in Colorado.