I can also second the idea of going to Paradise beach. I was there with my family last month and we had a great time! I would also recommend taking an excursion to Passion Island while in port at Cozumel. There are two ways to get there...by power catamaran (not a sailboat) or on the Twister. I have done both in the last month and highly recommend taking the Twister boat. It was a real hoot that can be enjoyed by anyone who enjoys getting wet. You do get jostled around a bit, but there were older folks on board (60's) who also seemed to have a great time. The beach there is beautiful and the trip is all inclusive, drinks and food included.
Good luck and have a great cruise!