It's a great island - wonderful beach, warm water, good snorkeling (not the best in the world, but very nice).
There are usually some excursions that have a charge - Banana Boat Ride, Paddleboat rental, parasailing. You can also hike (walk slowly - it's usually very hot) around the island. There's a bbq hut with great burgers, chicken, salads, etc. which is free and a bar hut that charges the same as onboard drinks.
Being the lazy person I am, I usually rent a $6.00 a day float and stay out in the water just watching the beautiful scenery. It's a thrill to see that huge ship anchored in the distance - a float is most relaxing and my idea of Paradise!
About the snorkeling, go all the way out to the point. You won't be disappointed, that's where all the fish hang out. I saw thousands of fish out there. Got a little freaked out when they started nibbling on my fingers. Snorkeling equipment is available for rent, but a little pricey. We always take our own.
The Island is very nice.
Here's a link to some pix of Zuiderdam at Half Moon Cay. On the left side of the web page, there's a link to pix of Half Moon Cay (Zuiderdam - Eastern Caribbean). Great place. I didn't know it while I was there, but they have some lockers in an area up in the plaza area, if you get tired of carrying around a load of stuff and want to go for a long walk. Don't forget the sun screen, or else you will get cooked.
If the link isn't hyperlinked, just copy and paste it into the address line on your browser.
We walked down to the area with the cabanas and just laid around on the hammocks. There was nobody else around us, and we easily had our choice of lounge chair or hammock or playing in the sand/water.
Planning ahead, we took a small backback with boxes of cereal to munch on (from breakfast that morning), bottled water (brought from Fort Lauderdale), a good book, sunglasses, camera, beach ball and inflatable air mattresses (from home). After embarking on a Sunday, spending the next day on HMC started our vacation off just right!! I fetched lunch for the two of us later in the day, as the walk was not that far.
We had planned on HMC to be our "beach day" since we had other excursions planned at each of our other ports, and this was our only opportunity to be total beach bums. Sure wish I was back there now!!!