I have not done any excursions off of the ships for Jamaica, but have stayed on the island 3 times. I definately recommend Dunn's River Falls. They are alot of fun. If you find the climb too hard you can always walk to the side and get off. A guide actually takes you up the falls. He will carry everyone's cameras so they don't get wet. Just give him a couple of bucks. At the top of the falls there are plenty of little shops that you can bargain for tchatchkes. There is also a little food stand that you can buy meat pies and fresh fruit shakes.
On another note. If you want something a little more relaxing I am not sure if your ship offers the Martha Brae River Rafting tour. This tour is a relaxing ride on a long bamboo raft down the Martha Brae River. Make sure to bring your bathing suit as you can jump in and take a swim. This is definately a relaxing cruise as 2 people can fit on the raft as your guide uses a pole to travel down the river. Again, along the way, you can buy little things. You can also get soda and beer and some food stuffs along the way.
As always in Jamaica, do not show alot of cash as they will follow you.