Rent a bicycle (near pier 39 or so), drive it across golden gate bridge to sausalitos, get some food, return with the ferry.
One time doing this (i live in the area), i met someone who said he was just there for just 24 hours, passing through from the UK to hawaii.
Haven't been downtown SF for a while, if prices for a hotel where unreasonably low (as in: not horribly overpriced), then its probbably because of the central subway construction project.