I know the LAPL excursion is on Grand Turk, and you can sign up for it by signing into your JC account. There is also going to be kickball at that stop. No need to sign up or pay anything. It will be right at the port and you can play if you want to. Or cheer. Or heckle. I prefer heckling.
I'm not sure what the MSC sanctioned excursions will be. I know that you can sign up for them when you get on board. Cruise Critic is a really great resource if you're trying to find something on your own. They have really good message boards that I have used to find tour giudes in the past. Here's the one for GT http://boards.cruisecritic.com/forumdisplay.php?f=385
They have a guy that rents jeeps and golf carts and lets you go on your own. Nathan's Jeep Rentals.