I went a couple years back to Playa del Carmen and rented a jeep and drove down the coast. Some awesome scenery, seclusion once you get a few miles away, and some great snorkeling. The beaches are like something you'd see in a movie (as long as you stay away from Cancun). The beaches on Cozumel were cool, too, but kind of crowded when we were there.
The best day that we had was when we found a great little beach bar, got some food, drank beer all day, and found a local who sold us a joint. We sat on the beach for about 8 hrs and mingled with the locals and just relaxed.