Closest to you if you're actually in San Diego is Corral Canyon. Lots of fun out there, Broncos Peak is a fun little trail.
It's a good drive from SD but Calico is A LOT of fun. This time of year there's runs being planned to there just about every weekend. Come spring time it'll be Big Bear season again up in the mountains. I've got a couple buddies going to Ocotillo this weekend, I'll have to skip that one though.
There's plenty of stuff around, just gotta keep an eye on the forums and see where everyone is going.