Hi,
We recently stayed in Playa Del Carmen for a week in August 2011, its about 40 min south from Cancun. The beaches were pretty great there, although I know the beaches at Cancun are better developed (and more crowded). Also, the shopping areas in Cancun are much larger along the lines of huge malls.
Nevermind what the naysayers are saying about Mexico. In these regions of Yucatan and Quintana Roo, it is extremely safe. In addition, no other place compares from a price point. My wife and I each got RT tickets from Baltimore for 300 dollars. We stayed at a resort called the Royal Haciendas. We used taxis to volley between hotel and downtown PDC (12 U.S.), the ADO bus to Tulum downtown and ruins (5 U.S.), a rental car for a day in Cozumel (35 U.S.), a rental car for a day trip to Chichen Itza (50 U.S.),and an ADO to Cancun (5 U.S.)
The best snorkeling in the area is in Puerto Murelos between PDC and Cancun. There is a protected reef there that is amazing to see. Book through your hotel, should not be more than 40 U.S. per person, all equipment provided (we took our own gear). For scuba, cozumel is the best.
Have a great time, don't be afraid to explore. Its safe, cheap, friendly and every business is ready to take your price.