I'll go in your place!!!
You don't have to take the ship excursions...as a matter of fact, IMO you are better off booking your own excursion. But the one thing they stress if you book a ship excursion is the ship will wait on you to get back..if you go out on your own...nope..they don't wait and its money out of your pocket to get back onboard!
I'm all for misskitty's idea of layering your clothes rather than going and spending a fortune on clothes you won't wear again for 2 reasons (1-you live in Florida and 2-your weight loss surgery!). Every cruise I've went on, I've bought items on eBay and did well.
I bet in San Francisco there are trolley rides that you can take that would show you highlights of the area and that maybe you could hop off and on. That's what I would recommend! We did that when we docked in Key West once...we only had 7 hours there and we took the trolley tour that took us all over the island and had a tour operator that told us what we were seeing and then you could hop off and on all you want. We wound up riding around the island one full time and then picking spots to stop at.
I'm sure you will have a wonderful time....I, myself, am very jealous of you!