This is a highly arid region much like that of southern Arizona. Take alot of water and you can find some awesome desert singletrack and some radical rock playgrounds.
How to get there:
From San Diego: Take Rt 8 east until you reach Rt 67 north. Take Rt 67 until it turns into Rt 78. This should happen at the town of Ramona. Follow Rt 78 west until you get into the park.