Has anyone done the excursion to Chankanaab National Park? The dolphin encounter? Two of my kids are not old enough to do the full swim. Is it worth it? Can we stay long enough to do the zip lines and snorkel? Will we be able to add on things like this when we are there or do we have to book them in advance? Is there a set time to head back to the ship or can we head back when are ready? Thanks!