There are tons of things, which I am sure some residents will tell you about.
When we were there with two children many years ago, we planned a very quick visit.
We parked at the underground garage at Union Square-- then walked down through Chinatown to the Maritime museum and Fishermans Wharf, where we sampled the seafood, then we took the cable car back uphill. It was all planned with downhill walking.
There is MUCH more, but this was a snapshot of SF.