Just saw the priceline.com advice above and the Park Hyatt is, indeed, a great property especially at those prices.

There is a fairly recent thread on the SF Hilton on O'Farrell Street in the Hilton Forum that you might want to check out.

I am not objective because I was born there, but San Francisco is a fantastic city with millions of things to do in and around the city. A fun day can be had by taking BART (rapid transit) under the Bay and sepnding the day in Berkeley - the UC Berkeley campus is beautiful and there is lots to see and do, including some great restaurants (plus, when it's foggy in San Francisco it is often sunny and 5 to 10 degrees warmer just 20 minutes across the Bay).