The first cruise we took with the kids was a 5 day out of Port Canaveral to the Bahamas which we combined with two days at Universal Studios. None of the kids had cruised before so we wanted to see how they did. Last year we did a 7 day western carribean cruise with three of the kids and we all had a blast. We did one excursion in every port and two in Cozumel.
In the Bahamas we did snorkeling, parasailing, wave runners and the dolphin swim.
In Grand Cayman we did the Sting Ray sandbar/snorkel trip, in Belize we did the cave tubing, in Cozumel we did the Sea Trek Helmet Dive and the Sea Lion Swim (both at Chankanaab Park where we spent the day), and in Costa Maya we rented a golf cart and went to Majahual and spent a few hours at the beach and just wandered around on our own.