First off, great web-site! This really appeals to me because I have a family with toddlers, where I cannot do some of the more extreme activities that I used to do but still love enjoying the many beautiful places the Bay Area & CA has to offer.
Something that excites me is Urban hikes/walks/bike. I like them because they are super environmental friendly, and easily accessible both location wise and economically.
A great starting place is the Golden Gate Youth Hostel. From there you can walk thru Crissy Field, over the bridge and then into the Headlands. Once in the Headlands one can camp, stay at a hostel, or live it up in Sausalito. Extensions of this journey can take you too Angel Island via the ferry or continue on to Stinson (not sure of the exact route(s).
Another idea is to hike from the SF hostel to Ocean beach. There is camping on the Presidio. While never doing so, I am sure there are other accommodations near Ocean Beach.
I bet there are some really fantastic walks in the East Bay. Perhaps going form Berkeley to Orinda.
Ok, just some thoughts. Cheers!