One of to the best places in Utah is Fish Lake. It is at about 8000ft elev. and is not too hot. You will have thunder storms that time of year. We have been there in Aug and it has been in the 40's up to the 90's. As for horse back riding, there is an equestrian only campground there also. It is tucked back it the trees and each site has corrals, hitching posts and plenty of room for the largest rigs. It also has a group site and rest rooms. It is maintained by the Utah Back Country Horsemen's Association. Very nice.
The lake even has boat rentals, pontoons the works. I have been going there for 45 years ( First went there with my parents when I was 3) and the people are as nice as they come.
FISHLAKE.COM and WWW.FISHLAKERESORTS.com
As for Colorado, the Durango and Silverton area is great. In Durango ride the train, go to Silverton, there are lots of back woods trails to explore. Go see Mesa Verde Nat Park and camp there. Visit the cliff dwelling / Indian Ruins.
Good times