You will love the cave tubing and Stingray City. These are the best things to do if you have never done them before. I would not book either of these through the cruise ship. Being a family of 5 you can save a lot of money booking them yourself.
Go to www.grandcaymancruiseexcursions.com
Look at the Buccaneer Stingray City Excursion only $39. We did this and loved it. You can also book your cave tubing on this site for $60 per person. Have fun.