The Sierra Gorda Biosphere Reserve, located in the state of Querétaro, is the most ecodiverse Reserve in the country, and has been awarded UNESCO World Heritage status. Sierra Gorda Ecotours, developed by the Grupo Ecológico Sierra Gorda which oversees environmental conservation efforts, provides the opportunity for visitors to enjoy the biosphere while ensuring the conservation of biodiversity and contributing to the livelihoods of communities, who operate the three lodges. Sierra Gorda was the 2006 Destination Award Winner from the World Travel & Tourism Council
More info at SierraGorda.net

Spend two weeks playing with crocodiles on the Zambezi River. Spend the morning setting crocodile traps and spend the afternoon measuring and tagging and counting crocodiles. This is a beautiful area and the schedule allows plenty of time for birdwatching and fly-fishing.
14 days for $2849. at Earthwatch