The Turtle Rock Trail at the Vedauwoo Recreation area is a blast. Vedauwoo exit off I-80, go to the recreation area and look for signs pointing to the trailhead. Ride it one way, then turn around and ride the other way. Also, if you like to explore, the main roads lead to lots of other less-used roads and trails. You can ride here for days on different trails.
Campsites are all over the place.
These trails are absolutely the best. If you are a beginner, go with someone who has ridden them before, as you will need a little technical help. Several portions of the Headquarters trail are fairly difficult and technical. Lots of rocks, some steeps, and big mosquitos in July. That is where the downhill is full of tough rocky sections.