A decade ago, some mountain bikers decided to create a single-track paradise for themselves on private Estrella Mountain Ranch land slated for development. They called it Fantasy Island North Singletrack, in honor of Tucson's Fantasy Island biking area. Friends started coming. Unlike skateboarders taking over a strip mall, they didn't get kicked out. Instead, signs were posted for each of the many trails, maps were printed, the secret got out, and the public got turned on to what now resembles a mountain-biking theme park. No purchase is required; just drive west on I-10 to Estrella Parkway, go south 10 miles to Weststar, turn right, park at the school, and bike another half-mile down Weststar to the trails.
Nothing there is too extreme, but some of the trails have sections that are difficult and potentially dangerous. Make sure your brakes are in working order. Someday, the whole area probably will be tamped down under a layer of single-family homes, so load up the bike and get out there while you still can.