For people in the South Bay area, you can head down to Fort Ord for a series of 9 weekend races (3 races are over already). These are put on by Central Coast Cyclo Cross (CCCX) and you can find a schedule of events here. There is racing for all classes and ages and from what I understand it’s a very fun series. Entry fees are $35/event.
Here’s some more fun for Sacramento area racers. There is a series of events at Granite Bay (NE of Sacramento) including several Friday Night MTB races, some Duathalons and mini-Triathalons. The races are put on by TBF Racing and there is a schedule if you follow that link. There are 3 MTB only races in June for all classes and age groups. Entry fees are $20.