Irish coffee at the Buena Vista and then rollerblading through Golden Gate Park. That was my first date 15 years ago with my now wife
Bring layers. It could be 80F or 50F and damp and windy ( like home?), no way to know.
The museums in Golden Gate Park are fun. The aquarium. The exploratorium if you have kids or are one, down on waterfront. The Marin headlands have good hiking and some old WWII leftovers for a bit of history, a mostly complete Nike missile site. Don't forget to walk across the Golden Gate. The list goes on....
Be prepared to part with your money, it's a pricy place to do anything but that's no excuse not to.