Well...it appears not many people from CO are on here...
I honestly don't know of many good places to trail ride here, except for local areas. Garden of the Gods(google it-you can't miss those big red rocks!), in Colorado Springs, has certain trails, and rather long ones through scenic areas(it's quite rocky and mountainous and wild if you take these trails) that you can trailer to, and then take trail rides from. Or you can sign up for trail rides @ Academy Stables(right next to Garden of the Gods- but you'd start the trail ride from there), if you don't have your own horse. And places up north, Near Estes Park, or Rocky Mountain National Park(also all mountainous) will have trail rides too- Estes Park is right outside Rocky Mountain National Park, and although I've never done trail rides up there, I've only driven up and hiked a lot, I would bet there would be at least one trail ride place up in that neck of the woods. Woodland Park/Divide, would probably have trail riding areas too, as it's in Pike National Forest. And south-west, near Canon City, Montrose, Trinidad, etc, that area has a few national Parks there as well- around here, with national parks comes other tourist traps, aka- trail rides, etc. So that area more than likely has something like that too. Check out Black Canyon of the Gunnison National Park, Mesa Verde National Park, and Great Sand Dunes National Park- it's a very mountainous area, right along the Continental Divide and the Rocky Mountains. And then look for local attractions around there- should find trail riding places as well.
About Garden of the Gods/most trail riding places around here- If you are an experienced horse person, or know more than the average bear about riding- do your research before settling on a place to go trail riding from. Most places around here, assume that if you aren't from CO/TX, you don't know how to ride a horse, just a heads up.
Hope that helps. I've lived in CO for 13 years and been all over.