Enjoy a vacation while doing good.

Organizations like Earthwatch Institute www.earthwatch.org/expedition/
offer the opportunity to go to the Amazon and explore by riverboat.
These trips include helping to save the endangered leatherback turtles on the beaches of Costa Rica.
Biosphere Expeditions, www.biosphere-expeditions.org is a non-profit
which helps bridge the gap between the scientific community and lay
people who are interested in contributing to a wildlife preservation.

Even sites like www.travelocity.com provides information and links
to vacation destinations where you can perform volunteer activities.