Hollister Hills and Prairie City, near Folsom are two of the State Vehicular Recreation Areas, that are located in NorCal. Haven't been to HH, but Prairie City has steep hills to climb, and obstacle courses. Has an O.R. track, but the hard clay soil and deep ruts make it brutal at higher speeds. Sort of small ... you only need half a day there to do/try everything.
Up in Lake County, BLM's OHV area called South - "Cow Mountain", looks like a large area.
http://www.blm.gov/ca/ukiah/cowmtn.html I want to check this area out this Spring.