Neely,
It's too late now, but My family took that exact same cruise at X-mas time 3-years ago. The best Shore excursion in Jamaca is a 35 mile MTN Bike ride. They take you by bus from the ship up to the top of a Mountain, and then you ride down to the shore and end at Dunn's River Falls.
The ride was GREAT! Downhill all the way. A combination of Dirt roads, and two track jeep trails.
At the end of the ride, the ride guides opened up a cooler of Red Stripe Beer! Then we climbed the falls and headed back to the ship.
Ahhh. What a great trip. I hope you're going scuba diving in Grand Caymen. It's some of the best diving in the world! Enjoy! ;)