We did Chankanaab! We got off the ship in no hurry, walked down the dock, went left and picked up a cab to Chankanaab. It was $10 for all of us in the cab (not $10/person). There were four of us, me & DH & DD(3 yr) and another guy who heard us talking at breakfast and saying that we were going to CNP and asked if he could come with It is about a 10 mintue ride. Admission to the park is $10/person and children under 7 are free. You could spend the entire day at CNP. It is awesme!!!!!!!!!!! There are plenty of restrooms, and places to get a drink (even though you don't want to drink anything other than SEALED bottled water). There are chair-things under straw umbrellas all along the beach. There were no shortages of places to sit on the sand, and plenty of stairs down unto the water. No spot was a bad spot. You could rent snorkel equip. for $5. (we travel with our own) The snorkeling was excellent!!! Water was warm. Fish and underwater life were plentiful. There were iguana lizards everywhere. There were small shops and fake ruins and an agricultural tour and diving caves.
On the way out, we bought a t-shirt for $10. So, in total, we spent $40 for our day. The cab (we split with the guy who went with us), entrance into the park, and a t-shirt. We spent 4 hours here. We would do this again!
Then, we went back to town and we went to C & Cs for a few hours. Back to the shipp to clean up and back to C & Cs till 10:00pm!
It was our favorite port!!!! Let me know if you have any other questions, I would be happy to answer.