Ft. Campbell has a nice riding stables that is open to the public on the weekends. Riding is $10 an hour. There are NO guided tours, you take your horse out on your own over endless wooded trails. Don't worry about getting lost... if you get turned around your horse will take you back (as he wants to get back to the food!).
Biggest thing is to be COMPLETELY honest about your riding skills!! Watching a John Wayne movie does NOT make you a rider! The "experienced" horses are just that, and if you don't know how to handle them ... it could be a long walk back to the barn.
There will be long waits on weekends falling after the 1st and 15th of the month.