Steamboat Springs? That would give you access to the mountans, plenty of hiking, skiing, rodeos, and the price range isn't astronomical.
Depends on what you're looking for and what you consider mid-sized. Steamboat has about 10,000 residents.
If you want to go larger, Colorado Springs is more mid-sized and has mountains not far off. It's a bit more suburban.
Should add that while 10,000 residents doesn't sound like much, they do have a lot more amenities/shopping there than you might otherwise expect given the ski resorts.
My wife and I stayed there for our honeymoon in 1972. Great ride through Rabbit Ears pass. We almost built a winter cabin in Estes Park.