1) You can save money by not doing an "excursion" and instead doing a little exploring on your own. Do you like beaches? If so, both islands have very good beaches near the docks. In the case of Grand Cayman just take a taxi to Seven Mile Beach. For Ocho Rios, you can walk (see photo below). I think the name of the beach is Turtle Beach, but I am not sure.
2) Dunn's River Falls is very popular in Ocho Rios. Check the port section or the Internet for private tours. Private tours cost less, but makes sure you get back to the ship on time. The ship will not wait for someone on a private tour.
3) I enjoyed the river tubing in Ocho Rios. See photo below.
4) Very popular in Grand Cayman is Stingray City. Captain Marvins gets a lot of good reviews. Again, that is a private tour, so you have to make sure you are back to the ship on time. Other popular places to go is Hell (it is the name of a real place on Grand Cayman) or the turtle farm.
Photo of Ocho Rios. The beach is marked, cost $1.00 to get in (as of Aug 2004) and is free of street vendors ...
This is the beach in the photo above ...
And this is why there are no street vendors ...
This is a photo of the river tubing. The tubes have wooden bottoms, so your bottom does not get hurt if you float over a rock ...
That is my knee in the lower left of the photo.