A good centrally located garage is at 5th st & Mission. It's right next to Metreon, across from the SF Shopping center, 2 blocks to union square and BART, and if you can put your bags in the trunk, should be no problem parking.

Stop by the shopping center, have a good meal at Sodini's (Grant & Green), or the worlds best burger at Bill's Place (24th & Clement), or have a steak at Alfreds.

MOMA is right next to the garage too if you're into art. In any event, the garage is pretty safe and there's lots of cabs there on Mission. And the parking is 1/2 the price of the pay lots on fishermans warf.