My standard recommendation: rent a bike. Rent one in the Wharf, and once you've done so, get the heck out of the Wharf. Ride trough the Presidio to the Sausalito ferry (i.e., across the GG Bridge). Send me a PM if you're interested, and I'll give you the details.

See http://www.blazingsaddles.com/rides.html (there are other rental shops, but this is the only one I've heard of).