Frank G. Bonelli Regional Park is an expansive park system complete with reservoir, swimming areas... and a surprisingly good trail system. However, it does cost $10 to park in the park, but there is a Park and Ride lot located just outside the entrance near the off ramp for the 57 Freeway. You can easily park your car there and ride into the park, getting access to the trails for free!
While at first the trails appeared to be just wide, gated gravel roads, as I continued around the park I started to find more and more singletrack. The trails were entertainingly rocky in places, and uber fast and smooth in others.
Despite having a trail map, I had issues following the designated route in places. As I was nearing the end of the loop, I took a wrong turn and ended up on a different trail that took me out of the park. It conveniently led me very near to where I had parked at the park and ride, so in the end everything worked out!
One word of caution: these trails are very exposed, and the majority of it has absolutely no shade or respite from the blazing sun. Ride early in the morning, or during times of the year when the temps are cooler.
Park at the park and ride outside the gate.
http://www.singletracks.com/item.php?c=1&i=5480