Can't help with the horseback riding. I like my bones in one piece
I had horses when I was younger, been stepped on, kicked, bucked off, rolled on, etc. I've had my fun.
I highly recommend Dunns River Falls, though. It's great, the water is cold, but feels great while you're climbing. You know what to expect at Stingray City, so why not enjoy some different sites in Grand Cayman? Have you been to 7 mile beach? How about Rum Point? Maybe do the horseback ride and then go to 7 mile or shopping.
I like to check out a new place or excursion when I revisit an island, but that's me. The only thing I do over and over is Trunk Bay and Champagne Reef, my 2 favorites, but even then not everytime I visit.
Bet this wasn't any help at all