I have a couple of friends who volunteer at the Museum of Natural History -- and love it.

http://www.amnh.org/

One is a retired psychiatrist and the other a retired social worker -- the Museum gives terrific training. One friend says it's probably an advantage not having a science background b/c you can more easily understand questions from people taking the tour.

I'm considering volunteering at the Rubin Museum (art of the Himalayas) -- not sure if they'll take me, but I volunteered for several years at the Met, so that might be a plus. I do have an interest in the art and it's within walking distance of my apartment.

Another idea is a group which helps newcomers to the US get acclimated -- http://www.intlcenter.org/volunteer.htm
I have a neighbor who volunteers here and really enjoys it.

I've done the food pantry and soup kitchen work and will probaly spend a few hours each week doing it when I retire but I'm also looking for something with a more "cultural" bent.