A question about volunteering...

My friend wants to do some volunteer work over Christmas break, and asked me if I was interested in joining her. (Of course I said yes! )

I was wondering if any of you know of any groups or organizations that have volunteer/ vaction things. (Aren't I so articulate?! ) That is, a trip we could take for a week or so that's run by an organization that is involved with volunteer work. So it would be a trip to another place (which always equals vacation for me) where we would do some humanitarian or volunteer work as the primary reason for our trip.