Hi, welcome to cruise-chat and congratulations on your first (but not last) cruise! I always do a google search for the ports of call. Booking on line is alway cheaper than going through the cruise lines. Otherwise, book at the excursion desk on deck 4 as soon as you can. Some of the popular excursions fill up fast. They have a lot of information at the desk.
I wish I lived as close to a port as you do. I live in Wisconsin and have to paid half the cost of a cruise in airfare.
I've been on 29 Royal Caribbean cruises and know you're going to love the Mariner of the Seas. The "Voyager" class ships are the most awesome ships on the seas. This being your first cruise you have no way to compare any of the ships. There is a big difference going from any other class ship to a "Voyager" class ship. Have a great cruise!