Conundrum hot springs is the most remove hot spring in Colorado. However it is very popular trail and usually full of people. It is located in Maroon Bells Wilderness near Aspen.
Be ready for 8 miles uphill with almost 800 m of elevation gain. But this doesn’t stop people from hiking there, partying there and staying overnight.
Because its popularity Forest service is considering establishing permit system for Conundrum. It should start from 2018 and permits for overnight stay should be available here: https://www.recreation.gov/