Here’s a great resource: Travel With A Challenge has all kinds of articles and links on volunteering abroad, especially for families and mature travelers. I like the way the article, “Volunteer Vacations are Contagious,” by Mary Jo Pehl, deals with the transformative nature of volunteering.
“Everyone is idealistic at first on these trips”, Ricki reflected. “Then you see real life with real people with real problems on their own territory.”
Bonus links at the bottom of the page go to Voluntourists Without Borders and The Land Conservancy.