re: Playa del Carmen excursions

Rent a car or three depending on how many people your group is. It's like $20/day for a 4-door standard, and you could probably get them down on that if you're getting several. Swing by a store and pick up beer. Head north a half hour to Puerto Morelos. Hang out on a clean, friendly beach and eat ceviche prepared off the boats.

Playa del Carmen is a tourist town, and Cozumel is like PdC, only an island. Tulum is awesome if you can catch it when it's not crowded, otherwise good luck finding a spot on the beach there. There are also a hundred different cenotes off the main highway outside of PdC where you can chill away from the crowds.