Dos Palmas - Tours from Playa del Carmen to the Mayan community of Dos Palmas, featuring adaptations of traditional ceremonies. - http://www.dospalmas.info/
The Vagabonders' Treks - Offers day trips from Playa del Carmen to the Mayan ruins, ecological locations and swimming sites. Includes itineraries, schedule and prices. - http://www.vagabonders.com/
Caribbean Coast Travel - Offers information on ground transportation and some lodging, with local information and images. - http://www.caribbeancoasttravel.com/