You will have a great time, we have been to many Caribbean islands once, St. Martin is the one we come back to time and again, and we are amatuers compared to many people come back year after year, it is that nice.
Yes to renting a car - there are many beaches and having a car lets you explore them. Mullet Beach would be a must visit, IMO.
As to Creole Snorkeling Tour - we took it, but it is very weather dependent. If it is overcast or windy, it may not be that great
Calmos is nice, but other options are a table on the porch at Ocean82, a bit more upscale, or going more downscale than Calmos, one of the Lolos nearby .
We don't go hiking, but walking from one end of the beach to the other, like on Orient, can be a decent walk, and others like the hike in to Happy Bay.
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