The Sebastopol Senior Center is located in the downtown area -- on High Street next door to the library and a couple of blocks away from the Sebastopol Center for the Arts and Ives park. It is a very active place with various exercise, discussion, and interest groups, games, dining and a transportation program. There are activities taking place there all day every week day. You can find more info here http://www.sebastopolseniorcenter.org/
I co-facilitate a women's group there (the women range in age from 67-90), have taken a couple of different exercise classes and have attended a couple of gay and straight dances there -- it is a fun and lively place. Note: I am a seniors real estate specialist, am familiar with centers in other towns too and with housing and services specifically for seniors throughout the county -- I invite any other questions you might have that I might answer.