I'm pretty sure these lockers have been out of commission since 9/11. You can either check the bags or take them with you downtown.

You can get there via BART (45 minute ride), SamTrans Bus(25 minute ride), cab or shuttle (25 minute ride). The first two are the cheaper options, though your limited to their schedule.

Whichever option you choose, your time in SF will be pretty limited. If you had a specific restaurant in mind, I'd suggest taking a cab there and back, unless it was close to BART or the bus terminal, especially if you're going to have bags with you.

You could always go to a nice hotel near the restaurant and check your bags for a few hours if you don't want to take them with you into the restaurant.