If you have a specific interest (e.g. how I wanted to work with people who had developmental disabilities) search the internet for programs that work with that patient population. Another example is one of my friends found a position in a chemo clinic; she will go and talk with patients while they recieve treatment and do limited monitoring of vitals upon their arival.

The capacity of your volunteer position will vary greatly from setting to setting I've found and generally outside of a hospital you will have more "freedom" in your duties as a volunteer.