If you are looking for a great Fall hike to an alpine lake then Gibson Lake might be for you. Great fall colors on a challenging hike.
The Timberline Trial hike is a shorter hike to a beautiful lake near Leadville. The big lake is one of my favorites with many areas to enjoy.
The Burning Bear #601 trail is a beautiful trail on Guanella Pass that walks through the woods, near a stream and features an old log cabin.