With teenage boys you may want to try a Dune Buggy beach & snorkel combo or an Eco Jeep Snorkel adventure. We did the 4x4 jeep two years ago on Cozumel and had a blast with our teenage daughter. Lots of fun.
Grand Cayman has Black Pearl Skate Park for skateboarding, and Black Pearl Surf Park for boogie boarding and surfing. We just got back recently from Freedom OTS with the Flowrider (really cool). One of the regulars, a guy in his 50's, tried the skate park and had great things to say about it. There's also a mountain bike adventure along the coastline or a Jeep and snorkel adventure.
If you go to RCI's website, check out all the excursions for the Ports you're visiting. Click on the drop down menu for "Before You Board / Shore and Land Excursions." I'm sure you'll find something that'll work.
I'm Steve Cooper. I love Royal Caribbean, and recently discovered NCL and love them too. I've also cruised Princess and HAL.
NCL Pearl Alaska Glacier Bay
RCI Liberty of the Seas E. Caribbean