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. After lunch in Valladolid, you'll visit the old colonial town, visit the monastery. and go for a swim at cenote Zaci.