Head south to Akumal. Very good chance to see large turtles there. Akumal is a Mayan word that means land of the turtles, or something close to that. You can take the colectivo from Playa del Carmen to Akumal for only $2 each way. A colectivo is a big white van that seats around 14 people. They run up and down the major highway all day long. They are much more convenient and much, much cheaper than taking the bus or taxi. Akumal is about a 20 minute drive south from PDC. It's well worth the trip if you like turtles. You can also visit Yal Ku lagoon while at Akumal. It's a short walk from Half Moon Bay (which is a short walk north from Akumal's main bay). The lagoon has very calm, sometimes brackish water. Some parts have lots of fish, mostly damsels and parrotfish. Honestly I wasn't terribly impressed with Yal Ku, but Half Moon Bay and Akumal's main bay are both worth a trip and have a high chance of turtle sightings (no turtles in Yal Ku that I've seen).