We have sailed the Destiny twice (great ship) and would recommend highly Nautilus snorkeling. We are beginner snorkelers, and had a ten year old at the time--they took very good care of us, snorkeled two locations--one near a reef if I recall, and another at "Christmas cove" I believe where Christopher Columbus spent his first Christmas, this also had a small white sand beach to rest on if you did not want to snorkel second site. They also had snack & water, plus caribbean Jimmy buffet tunes on the way back.
Second time--we booked on our own (carnival offers sometimes) a tour called the Thriller. A high speed boat that does wild turns and spins for about 40 min. Honestly fun first 10 min- (REALLY wish I had worn pool goggles) but after a while I got sick of being sprayed with salt water, drenched and no where to wipe salt water out of eyes. Sunglasses are very useless unless you have tight leashes on them. Near the end of tour, guide cut thru a wake and a HUGE wave smacked all of us. We were drenched. All of our belongings by the way were securely stored and watched by tour operator partner at the starting point. So great thrill ride, but must enjoy getting wet (and a bit cold--you know the air cools ya, and only way to warm up is the warm water again.) Great tour operator but for us, one time event only.
At the pier we quickly shopped for a snorkel mask for son, and I found in the window my best purchase, a floating for at the beach. The rim inflated and middle was net like and stored upon itself. I used it all week. In the PM we hired a local taxi to take us to Saphire Beach
--best beach of our trip. Nice shade area, chairs (noone ever asked for money), and good snorkeling near shore my son said. Taxis are readily available to take you back. (The taxi driver told us next time hire a taxi to catch the ferry we drove by and go to St. John's,). I'd easily go shopping near cruise ship and hit the beach here, and if time catch the tram ride up for the picturesque view--it is only a few blocks from ship, right by shopping area. The second time we did not bother to go into downtown to shop, so many stores are the same and ran out of time.
Dominica: there is shopping at port, more than cruise ship describes.
The Wacky Roller trip looked kinda fun.
Aruba--we did not get to this time due to hurricaine Emily changing our path. I would recommend renting a car. That way you can get to Palm Island Beach (drop off I think was "Aruba Grand Beach Resort), the Natural Bridge and shopping at will. There is a lot of shopping here and all with in walking distance. The shopping is also near famous bars that many people came swaying back from.
So much I could type--PM me, or post back and I can contact you if you have more questions. We know so much about logistics of San Juan, ship etc. - I posted some of our photos in gallery--do search "Destiny" -I am Suzy. http://www.cruisemates.com/gallery/view.php?id=2574