If south Orange County isn't too far south, I like San Mateo Creek campground. It's part of the San Onofre State Beach Campground, but is located a mile or two away from San Onofre. On reserveamerica.com, look up San Onofre first then drill down to the San Mateo section. Literally right across the street (Christianitos Rd) from the campground exit is the San Clemente Singletracks (look it up on geoladders). A few miles up the I5 you'll find the Ortega Highway exit, which takes you east about 30 minutes away to the San Juan Trail, a SoCal epic that has some challenging switchbacks and a bit of exposure at the beginning, but mellows into a long steady beautiful 100% singletrack ride - not to be missed. Or take the I5 norht a few miles to the toll road, hop on it for 20 minutes north to Aliso Creek Rd. and go sotuh 10 minutes to Aliso and Wood Canyons Park, a very mellow ride spot. if you have more time, take a scenic drive up Pacific Coast Highway past Laguna Beach to El Moro State Park, another mellow riding area, then after a nice ride, drive back down PCH and have brunch in Laguna Beach afterwards - we like The Cottage. Other area campgrounds include San Clemente State Beach and Doheny State beach. You can't go wrong with any of these, but I'd put San Onofre last on the list as it's a stone's throw to the freeway and is more of a parking lot type campground.
