This small island off of eastern Mexico is a pretty tourist stop on the Caribbean Sea, with a beautiful colors and tons of unique little kiosks, especially near the sea. With a maximum altitude of four feet above sea level, it is the lowest-lying land in Mexico. It probably going to be jammed with tourists from the passing cruise ships, but the island maintains a distinct Mexican feel.
Check out one of the Dolphin Adventures options on the island. You can have the opportunity to ride, swim and play with real dolphins, right in the water!
The sun burns brutal this close to the equator, so keep sunscreen handy. Bear in mind, though, that you cannot wear sunscreen in the water with dolphins, as they are tremendously sensitive creatures.