Located about 80 miles south of Cancun, Tulum is the only walled city the Maya built and the only one by the edge of the Caribbean. When the  Spanish first came upon the site in 1518, they recorded that it was as large as  Seville. The major construction is the Castle, a handsome pyramid perched on the  edge of a cliff high above the pearl-white beaches below. From here one can see most of the ancient city, as well as a magnificent view of the coastline.

On the way back you can visit fascinating Xel-Ha, a park and natural aquarium with four interlocking lagoons where swimming and snorkeling are superb. There are also jungle trails, a shell-strewn beach (not for  swimming, but great for exploring), and dolphin pen where you can swim with the  unique creatures.

The newest feature at the park is the Aquatic Institute, which offers snorkeling instruction with an emphasis on the identification and responsible handling of underwater flora and fauna.