Disney cruise with 4 yr old? Shore excursions to do?

Going on a 3 night Nua/castaway cay cruise- wondering what shore excursions are the best to do. We have a 4 yr old son who is going with us. I also wanted to know can we stick him in the child care and go on a shore excursion by ourselves (like something he's not old enough for) or do you have to stay on the boat when your child is in the daycare?