Hidden Villa

Hidden Villa is a working organic farm on 1600 acres in the Santa Cruz Mountains (about an hour away from Contra Costa County.) It's also a not-for-profit environmental education center.
Some of the many programs available at Hidden Villa include summer camps, workshops, concerts, and a hostel. There are miles of hiking trails for those so inclined, and in the spring, Hidden Villa offers the allure of....Baby Animals!!!
They also offer classes, a few are adult classes (canning, growing edible mushrooms), but the majority of the classes are either kid-friendly, or aimed directly at children, with themes like "Preschoolers on the Farm". Every weekend they have tours of the farm at 11 a.m. and 1 p.m., rain or shine. I believe these tours are free.
Parking is $5 and entrance to Hidden Villa is free.
Along those lines, Emandal Farms in Willits is also an environmental education center. They offer family camping in cabins along the Eel River, with all meals included in the cost of your stay. While there, you can milk cows, pick tomatoes, and gather eggs. If that's not your thing, then you can take a dip in the river instead. Sounds pretty nice to me.
http://www.emandal.com