Classic mountain bike single track that can be shuttled or done as an out-and-back.
The San Juan Trail is located off the Ortega Highway (Hwy 74) in Southern Orange County. It is done frequently as a long, 11 mile climb that is rewarded by an almost continuous descent of 3400ft. The out-and-back trail head is back Hot Spring Canyon Rd from the San Juan Fire Station. It can also be done, by those less inclined to ride uphill, as a shuttle starting at Blue Jay Campground. You will want a Forest Adventure Pass to park at either end. The trail is all single track and the views can be stunning. There are only a few really technical sections and the majority of the ride down is fast and flowing.
The Chiquito Trail is an adjoining trail that is also very enjoyable. On subsequent trips it would be worth checking it out as well.
Access to the trail is virtually nonexistent other than the trail heads so it would be smart to bring plenty of water and a tire repair kit/tools.