Montego Bay is not one of my favorite places, usually we just stay on the ship and enjoy having it to ourselves, although in recent years more and more folks are opting to stay aboard as well. But . . . I have done these things and found all of them enjoyable.
Dunns River Falls - the drive over to Ocho Rios will definately get your blood circulating but climbing the falls is a very unique experience. Be sure to bring either water shoes or an old pair of tennis shoes that you can throw away, as you need to wear shoes to climb the falls.
Snorkeling aboard the Jolly Rogers Pirate ship. We went out to a national park area and did some great snorkeling. The water is fairly rough in that area so if you are not comfortable snorkelling in open water this may not be for you. After we were aboard rum punch and red stripe beer was available. They then stopped at Margaritaville for a while so we could slide down the big slide, sample margaritas and hang out. You had the option of taking the pirate ship back to the pier or arranging your own cab ride home for those who wished to shop.
Bamboo rafting trip. This is very laid back. There is a pretty ride up into the mountains and you board this little raft for two that is poled down the river by your "driver". There were some very pretty sights along the way. At the end there is a "how to" lesson on making the perfect pina colada, complete with a local climbing the coconut palm to fetch coconuts for the drinks. We did see some beautiful scenery and butterflies and birds on this trip but it would be seen as horrifically boring by some.