We just stayed right at Great Bay in St. Maarten as well. We got off in the morning, went over to the beach on the water taxi ($5 per person all day and kids free). Spent time playing in the water, we rented jet skis, used the water trampoline etc. We had two chairs with an umbrella. Chairs were $10 for 2, $40 per 1/2 hour for jet ski and trampoline was $5 for the day. You talk to them and bargain it up, we got everything for $45. We then returned to the ship to shower, change and have lunch then used the all day water taxi again and did some shopping on Front Street. It was a really nice day. There are a lot of vendors walking the beach, we found it to be pretty annoying at some points, but as said above, a polite no thank you keeps them moving.
For braiding, my wife had it done there and they gave her the "today's special" of $1 per braid, did hear some people talking about $2 per braid. I talked with the girl and made sure she told me the most she would charge. She told me a bout 20-25 braids. I said fine but no more than $40 total. She said ok. It turned out my wife had 36 braids so $36 total. I just gave her the $40 and called it a day. If it were $2 per it would have been over $70 so just make sure up front you know what you are going be charged. Our table mates had two little girls with really long hair and they wanted to charge them almost $150 for the two. I remember them saying they finally settled on $100. So just know up front. But they did a great job.
So have fun.....