El Garrafon Park is located on Isla Mujeres, about a 30 minute boat ride from Cancun. We paid around $40 each for a package trip to the park. It included the ride, a buffet lunch at a very quaint, clean restaurant, and use of the park facilities (pool, showers, lockers, and some really nice hammocks in the shade).
There is a small charge ($2) for snorkeling. The reef is right on the shore and about 100 yards long. There are very many colorful fish to see of various sizes. I was lucky enough to have an encounter with are very large sting ray (6 foot wingspan).
A short walk to the southern tip of the island is a small Mayan ruin. The views from the path are very picturesque. The hammocks in the shade made for an excellent siesta.