I usually go off the Otay Lakes road in Chulla Vista. It's right near an airstrip where you can sign up to go parachuting. The area is split up in sections marked as "wild life preserve", so just watch for the signs and keep off the land. Boarder Patrol is in the area to open the gates if you get yourself behind one. For the most part, there are areas off the Otay Lakes road where you can enter.
If you get up past clairemont on the 15, there's a small area for offroading slightly to the west of the freeway there. Search for "airstrip" or "airport" and you'll find directions. If you don't want to drive very far, its a good idea. Otherwise, check out jamul; you can always find trails(try proctor valley)
take the 94 east out of rancho san diego. once you get into Jamul, turn right on Proctor Valley Road. take that all the way until it becomes dirt.
right before it becomes dirt turn right onto another dirt road. take that road around. since its raining there will be some mud and deep ruts to play in. if you take that road around there are some hills to climb and a few rocks.
if you follow a side road that heads toward the mountain there is a pretty steep trail that leads up and over.