Depends on your definition of fun.
I was out there for a week with my my BF (zenmonkie) and we spent a lot of time in the Castro district window shopping, eating at a few different places and more than our fair share of bars. If you have the opportunity to grab a bite to eat at the Squat and Gobble DO IT! It's a great little deli with a huge selection of meals all made on site and from scratch. There is always the touristy stuff like walking the Golden Gate Bridge, the Alcatraz tour, the Ghirardeli Chocolate Factory, the Presidio, riding the trolleys, China Town and of course eating all the seafood you can afford to on the Fisherman's Wharf. If you're in to leather or kink you'd be missing out if you didn't swing by the Mr. S Leather store. You could also tour the San Francisco Armory which is home to the Kink.com studios as well as an impressive historical military installation.
San Fran is such a vibrant city and there is ALWAYS something going on. Talk to a few of the locals when you get there to see what special events are taking place. We found out about this electronic music festival called the "How Weird Street Faire" on Howard Street and that turned out to be an awesome day. Listened to a lot of great music, watched all sorts of people having a good time and witnessed a lot of strange and amusing things.