Rampart is probably the biggest and most well known OHV park in that area. I haven't been in years - there's a lot to do there but the last few times I went it was really crowded. The MX, ATV and SXS crowd had grown a superiority complex and were giving us a hard time. Could be totally different by now.
Mt. Baldy is another close to COS. Haven't done that one but the youtube vids look fun.
Then there's Slaughterhouse Gulch OHV Park near Bailey, it's not very technical but a fun day trip. Alpine loop is awesome, Imogene Pass, Engineer's Pass, Ouray......there are so many. If you're looking for a straight up OHV park, other than Rampart, there really aren't any really great ones. Tons of passes and mountain roads. Plenty of vids on YouTube for you to check out. Not familiar with China Wall, I'll have to check that one out.
Check out the Colorado section of this forum and Traildamage.com (and other similar sites).