Be forewarned that the lines to get on the Cable Cars can be horrendous, especially on a Saturday morning. You might do best to plan SF for Friday afternoon evening (maybe even before checking into your hotel): go to the Cable Car Barn (per dgolds' suggestion) before it closes, then take a late afternoon or early evening ride on the nation's only moving historic landmark. Also allows for a dinner in town (lots of good choices, in all price ranges).

Then Saturday morning either sleep in, or do something down closer to your hotel or the wedding. Much less angst if you're in their neighborhood, rather than an hour (or more) distant and wondering how much time to allow.

And don't worry: you'll leave your heart here (everybody does)!