We went to the Bay a couple times while in SF. One time we walked around to a few places & another time we rented bikes so we could cover more ground.
It's a beautiful bay to walk around, take in the views & do some people watching. We saw people swimming in the freezing cold water and watched a bunch of kite surfers get their gear ready before they went into the bay to take advantage of all the wind SF has.
The land was flat all around the bay (besides the hill that gets you up to the Golden Gate Bridge) so it's easy for anyone to enjoy.
Even though you're still in the city it feels like you're in a park when you're by the bay.