Visit Ek Balam (Mayan for "black jaguar").
Ek Balam is probably one of the most beautiful Mayan ruin cities, though few buildings have been excavated is definitely worth a visit, if only for the well-preserved Frescos at the excavation site.
Shore excursion for cruise ships in Cozumel - 7,5 hours. + 1,5 hour for roundtrip cruise ship - Playa del Carmen!