Akumal Bay has good snorkeling but the water can get silty if it is windy. Half Moon Bay is about a mile north of Akumal bay, and I think the snorkeling is better, although it is rocky.
If you go to Akumal Bay stop at the Lo Ha beach bar for happy hour. This
webcam is mounted on the front of the dining area.
For great margaritas go to La Cueva del Pescador. The also have really good food.
If you are the least bit interested in Mayan history, be sure to visit the ruins in Tulum and better yet, take a tour to Coba. In my 2007 trip, I hired a cab for the day from Akumal to Coba and back, and it only cost around $50, but this trip the DW and I are renting a car for the first 10 days.
If you enjoy snorkeling, I would highly recommend taking a resort scuba course. Better yet, if time permits, do your classroom and pool training at home before you leave, and do your open water certification dives at one of the dive shops in Mexico.
Check out
LocoGringo for tons of good information on the Mayan Riviera. I always book with them when staying in the Akumal area. Their
forums also provide a wealth of information for activities in area.