I don't have much time for volunteer work right now (though I plan to do some when I retire in a few months).
I can tell you that in the past I got a lot out of my volunteer work at a shelter for battered women. It fills a real need. It can be frustrating to watch people return to abusive situations, but you learn that just as in quitting drinking, it takes many people several "practice" attempts to change their situation before lasting change is possible.