Hi MaryaliceZ, welcome to Cruise-Chat! We were in Montego Bay a couple of weeks ago. We took a cab tour because our friend wanted to see some sites. Dunn's River Falls is in Ocho Rios which would be a long ride from Montego Bay. It's not too late to book excursions. You can even book them onboard the ship. In Grand Cayman, I would highly suggest the Stingray City excursions. I alway use Captain Marvin's for that. You can save money if you book directly with a tour company on the internet. Here is a link to Captain Marvins if you want to take a look.
CLICK HERE
Hope this is of some help...
Sonny