Playa del Carmen! We were there last week. Dove with Reef Quest Divers - Steve is great! Since you have kids I would suggest looking at the Grand Porto Real (AI) - we have been members for about 10 years and love to place. Very kid friendly and the dive shop is about a 2 minute walk down the beach. I would also make a trip down to Akumal for snorkeling. We took a group of 10 to this protected area - saw turtles galore and my favorite - I snorkeled with a Eagle Ray for about 100 yards before she left me.