I have not rented bikes there, but I know that my mom and her boyfriend shipped their bikes out with them(they just spent a month out in san fran). They did a lot of riding down by the marina.
I also just spent a week with my older sister and her boyfriend(they live in the north panhandle) and I did tons of running through the panhandle, presidio, and golden gate park. You could definitely ride through golden gate park if you wanted to avoid all of the traffic of the city, and then ride out to the beach and ride along there.
I know I am not much help, but some of the fine ladies of the bay area on here should definitely be able to steer you to a good lbs to rent a bike from.

and beware of the death monsters!!(ahh i loved seeing this on the MUTs)