Another of the things to do in the area of Tulum is a visit to one of its sacred Cenotes. A cenote is a natural cave flooded with water that you can visit on a snorkeling, or scuba diving, tour.
The two biggest cenotes are: Dos Ojos and Hidden's World. Hidden World's is more commercial, and Dos Ojos is less developed. I visited Dos Ojos and can tell you that looking at the stalactites and stalagmites while swimming in a cave was an experience I will never forget.