I know there are probably other Bay Area shops that rent bikes but here's one I found. When DuckonWheels was here I rented one for her so she could do a TE ride with us.

You'd need a bike rack or means to get the bike back to S.F. but it's nearby, and they have some pretty decent hardtails and mountain bikes, they will even fit, swap out pedals and saddles etc.

www.summitbicycles.com