Dunn's Falls is fun but if it is crowded like it was for us (four cruise ships in port at the same time) it's a nightmare. I preferred the tube riding on the whitewater. If fact, it was one of the best things we did on the excursions. My family and I had a great time there! Suggestion, take a Trevor Hudlin private tour (google Trevor Hudlin and you'll find his site). He'll take you anywhere you wish to go. We did Dunns and the tubing plus he gives you a great island tour including Fern Gully.
Grand Caymen - Stingray city was GREAT! My kids loved that one and so did my husband and I.
Cozemel - Dolphin Adventure. BEST thing we did on the cruise. Swimming with the dolphins, hugging them, kissing them -- the BEST! The pictures they take are wonderful PLUS they video tape it for you. I HIGHLY recommend this one.
Have a wonderful cruise!