Hey Marco, I recommend the Lake Tahoe area about 4-hours drive from SF Bay Area. Northstar is a ski area that opens to mountain bikes in the summer. There's also the Flume Trail and other options on the Tahoe Rim Trail that can be done with a shuttle--flumetrailtahoe.com. Another nice ride is the Downieville Downhill in the Sierras just north of Tahoe--downievilleoutfitters.com or yubaexpeditions.com.
MTBProject has most of the trails and ride areas in central California posted. If you're in southern California I'd say check out Big Bear or Mammoth Mountain.