Hook up with Fleet 281 at their monthly fun sails on Tomales Bay. http://www.hobiefleet281.com
Fleet 20 has fun sails at Woodward Reservoir.http://22.214.171.124/~hobiefle/
Fleet 2 has fun sails in the Sierra Nevada foothills.http://www.hobiefleet2.com
Fleet 259 is resurrected and is attracting sailors to their events on the Central Coast.http://www.hobiefleet259.com
And Division 3 has a bunch of regattas that will take your sailing skills to a whole nother level.http://www.div3.hobieclass.com
If you're looking to do your own thing then Central Valley lakes are a good choice.
New Hogan Reservoir
Rio Vista - winds can be mighty strong though
Clear Lake - not Central Valley, check the algae bloom and lake grass status before going
Bay launches are not as friendly because public access sites don't allow camping, but Crown Beach in Alameda has good beach access if you have beach wheels and a couple strong guys to help get the boat back up the steep drop from the parking lot to the water. Also, learn to read a tide chart so you don't plan a day when the tide is really low.
Contact the Redwood City Marina for info on launching from there. It's supposed to be very good sailing there.