My favorite ride near Canon City. I prefer this loop in the counter-clockwise direction (which makes for less technical climbing), the ride down the Lion Canyon singletrack is narrow, steep, and fast. The climbing is on roads/doubletrack and you end by descending the 2.5 mile Lion Canyon singletrack back to the campground and your car. However, on this route finding the upper Lion Canyon TH to get down off the mountain can be tricky. Some eliminate this confusion by riding the loop clockwise - climbing the singletrack and taking dirt roads down off the mountain and back to the car. Either way, this is a great 2-3 hr ride (~15 mi) close to town.
If you don't want to deal with finding the upper Lion Canyon TH, ride the following loop in the clockwise direction, starting with a climb up the Lion Canyon singletrack and going right on Highline Trail Rd to the NF Rd 304 jeep road descent to CR 143.
For the counter-clockwise version, ride out of the campground to CR 143 and turn left. Follow this road, staying left through a Y in the road and the following intersection. After a couple miles of uphill, you come to a fast downhill section and NF Rd 304 on the left. Turn onto this jeep road and climb for around 3 miles to the top of ridge, where you will encounter the Highline Trail double track. Turn left and follow Highline past multiple, signed spur roads that end in the letters A-H. The upper Lion Canyon TH is at the end of the spur road ending in H, which breaks off to the left. I believe this is the last spur road before the Highline Trail road dead ends, so you'll know if you've gone too far. If you can't find the upper TH you can always back track the way you came, enjoying a fast downhill on the jeep road that you climbed to reach the Highline Trail. Following the directions above, however, it shouldn't be hard to find the upper TH on the counter-clockwise loop. From the top of Lion Canyon trail bomb down 2.5 miles of technical and fast singletrack to the campground and your car.