Join the Journal Square Neighborhood association. Lovely people. They have a speaker every month that might be arty, history or local issues. I find its a great way to meet people. Park your car in union city and pay so you dont hassle with it. Ride a bike around town or take VIA. Your best park is Pershing Field Park. You can get some green time volunteering there with the Pershing Field Garden Friends.
I dont know but your alumni association might have a chapter here. Get on the list for the Hilltop Association, too. They are on the eastern side of Journal Square. They have a facebook site. The city site has local parties, get togethers, new rules etc. I dont know your interests.
The local Sierra Club has many hiking, canoeing and naturalist tours. There is a group for everything here.