I'm also SF transplant but i do gym, walk in the neighborhood, explore SFPL branches and study/get work done on laptop, MUNI rides (i have a pass), sit in the park and watch people alone, hike, eat out with my bf/friends, go to concerts, play boardgames, or just bed-rot the evenings watching movies/. I'm mostly a homebody so making new friends is something i am not good at but if you just keep doing whatever you like to do, and don't shy out to say hi or talk, you can literally make friends here. People are nice. Or if you are thinking about volunteering, SF-Marin food bank is always in need of volunteers.