I'm a (kind of lapsed, unfortunately) Boston Cares member. They have versions all over the country though. Basically, it's one-off volunteering. You can sign up for as many or as few hours as you like and you're not obligated to do the same thing every time, though you can if you'd like. You can find your local affiliate here. I served brunch at an AIDS outreach facility, participated in a program that sends books to prisoners, and played Scrabble at an assisted living center, just to give you an idea of the variety of projects. (I'm not lapsed because I didn't enjoy it; mostly it's time and the stuff that piques my interest isn't nearby and I don't have a car.) But if you don't know what you want to do, it's a good way to try things out on your own schedule.