All the islands pretty much have similar activities. Shopping, City tours, island tours, snorkeling. Of course there will be other activities, but these are usually givens on any island. In St Maarten when you get off the ship and start walking down the pier to the shopping area there is a big billboard. It has pictures of different island tours you can take. It list the prices per person. The larger your group of people, the lower the price. Don't worry the drivers will be right there in all the ports to try and talk you into tours. Never pay ahead of time, always pay at the end of your trip.
If this is your first cruise I would recommend going mostly with the ship's tours. You might feel more secure knowing they will make sure you get back to the ship on time. If you do things on your own you need to allow plenty of time to get back to the ship.
Tours probably start around $25/$30 and up per person.