Congratulations on your anniversary and upcoming cruise! You have selected a fantastic ship. Here
are some pictures if you'd like to take a look. I totally agree with Rusty57 on Trunk Bay. It is a beautiful beach with a self-guided snorkeling trail about 50 yards from the shore. There's plenty of shade or sun, fresh water showers, a place to grab a snack, lockers, and a couple of shops (do your shopping in St. Thomas, though).
We just returned from St. Maarten and did a nice snorkel there with Snorkel Safaris, you could probably find them on the web at snorkelstmaarten.com. It was a great excursion. They take you by water taxi to their floating dock, have equipment, lessons, and a pool if you've not snorkeled before. The drinks were reasonably priced, as well as the snacks and food. The best part was, you could stay as much or as little as you like. They'd even drop you off in town for shopping at no extra cost. The operators were very friendly. The snorkeling in Grand Turk was much better, but it was a nice way to spend a few hours.
Of course, you guessed it, we snorkeled in Grand Turk. We did our excursion there through Costa, and I don't remember the operator's name. I'm sure Princess uses the same one, though.
Princess Cays is their private island, so it's a great beach day. They have a nice bar-b-que on shore, there's some shops, and of course, they have plenty of bars. If you walk through the fence in the back of the area, there are some local vendors with typical tourist items.
If you do go to the Crown Grill, make sure one of you try the Black and Blue Onion Soup, it is fantastic! I had the petite fillet and my hubby had the lobster. The petite fillet was much better than the one I had at Ruth Chris's before we left. My hubby left nothing of his lobster, so I assume it was ok. We had no problem getting a reservation, but we were on a 10 day itinerary, not a 7, so it might be a good idea to book ahead of time. The formal nights should be your first and last at sea days. That way you won't miss the lobster in the main dining room.
Hope you have a wonderful cruise and that you enjoy Princess as much as we do. Best wishes!