You know the child in question best. What do they like?
Me, I'm not one to spend money on excursions to start, and I'm even more weary of kids. My daughter is not an adventure nut nor is she keen on animals or wildlife (kinda like her mom in that regard). She'd be freaked out about seeing a turtle. I couldn't even get her to get an upclose look at the Iguanas that grace the port in St. Thomas.
In St. Thomas, we just took a trolley/bus to Magen's Bay. It was a little rough the day we were there, and my daughter lost her favourite pair of sunglasses in the ocean. HAd it not been for that, she would have had a great time. Great beach for building sandcastles!.
My kids are very happy just playing on the sand. When they are older, we'll opt for more adventure in our choice of excursions.