Do you work with a specific community or organization, or just take opportunities as they come up?
What type of volunteer work do you like to do? Do you like the stuff where you are just a warm body - someone to hold a sign or greet people? Or more involved tasks that are more like professional work? What about shudders door knocking?
These days just surveying for the national bat and dormouse monitoring programmes, but I have done quite a range of other things in the past including: starting and running a Green Drinks group, pagan prison chaplain for the Pagan Fed, direct action with Greenpeace, local wildlife group committee member, starting and running a toad patrol group, helping at an old folks day centre, running a tea stall and car-parking at a few green festivals, crash and bash conservation work with various groups etc.