Probably the most accessible climbing area in San Diego, Mission Trails Regional Park is the largest municipally owned park in California. At the center of its 7,000 acres just off the scenic Father Junipero Serra Trail are 4 climbing areas which provide a decent collection of sport and traditional routes. The cliffs are generally west-facing, which makes climbing possible throughout most of the year by arriving earlier or later in the day.
Many top-ropes can be set by hiking around or scrambling up a gully, and it's common to see a guided group or two on the weekend.
Do note that the gates are locked at sunset, so watch the time to keep your car from being stuck overnight.