If up for an adventure, last time in Montego Bay I took the "shuttle bus" to the Shoppes at Rose Hall and then walked to Rose Hall and took the tour. Pretty interesting.
In Grand Cayman, I like the sting ray excursion offered by Soto's Cruises
Stingray City | Grand Cayman Tours | Sotos Cruises Daily Since 1987
I have lots of pictures from both.