San Marcos:
La Costa trail network. Bring a 1.9 scaler and about 6000MAH of battery if you want to go to the top and back. Lots of steep, rocky, dirty fun to be had with a scaler.
Elfin Forest Recreation Preserve. Much smaller area, but more technical. And shady, so it's nicer in the heat. Good for a 1.9 scaler.
Escondido (next city east):
Daley Ranch. Lots of technical lines to explore so better suited for a 2.2 scaler or Capra / Optic type vehicle. Head to the Whisker Bay portion of the park. $5 to get in on weekends, free on weekdays.
Jesmond Dene park. This is just for bashing, in case you have some high speed offroad cars to bring. This track has room for 1/5 scale cars, and the terrain to challenge any suspension set up. Mostly hardpack with some deep sand. I run my UDRs here for scale San Felipe 250 action.
Let me know when you'll be around. I'll likely be available for some RC driving.