There's plenty to do either in SF or close by. SF has a lot of different districts, museums, parks, etc so you could easily spend all your time there using the excellent public transport. Or you could hire a car and explore the coast (either north or south), or head to the wine country, or the gold country & Yosemite. I've traveled there on my own and could happily spend longer than a week. Hotel prices seem a little high at the moment due to the strong dollar (based on what I've been looking at for my short stay) but I've never had a bad result when I've bid for SF hotels on Priceline.

eta, just seen that I've echoed some of crazy8534's thoughts - apologies, but then great minds think alike.