I have been to Jamaica a few times. Once on a cruise, once for a 10 day stay.
The falls are great but from MO Bay I agree with others that the drive would be horrible. Heck the drive from the Airport was the worst part of out trip. The roads are always beat up, tore up, and completly pot holed. And the drivers are just about the nuttiest I have seen anywhere.
I have not zip lined but as a sky diver this never interested me. I would do the Zip line because of it's location from the ship. As a matter of fact I am going back to Jamaica agian on the Liberty of the sea in Oct. Might just try it with some of my girlfriends that have never jumped from a plane.
Good luck and have fun!