Just got back from a weekend at William Heise County Park - this is in the hills to the east of San Diego, close to Julian. It is about a leisurely 2 hour drive from San Diego.
The park is very nice. They have sites that have partial hookup (electricity only). but they are crammed together - literally only feet apart. By contrast, the non hookup sites are spread out over a beautiful campground among tall oaks. These sites all had an asphalt pad, table and fire ring. Some of the sites are close together, but many have good separation from neighbors. Sites fees were only $15 per night. Restrooms were all clean and well maintained. There is a no cost dump site in the park.
You can book online here:
http://www.co.san-diego.ca.us/parks/online.html
There are several miles of very good trails around the park.
Note for dog lovers - all of the web sites, printed material, and park signs say that dogs are not allowed on the trails. We were told by the Park Ranger that this has now changed and leashed pets are now allowed on all trails.