Can't help you getting to the port, but having sailed on the Grandeur - you should have a great time.
RCI encourages passengers to complete the registration process online in order to expedite your embarkation (make sure you print out a hard copy so that you will have it at check-in as a back-up). You will need your sailing date, ship and confirmation # to do this.
I would suggest cave tubing in Belize, and snorkeling in Cozumel. Both can be done on your own, or if you prefer, through Grandeurs shore excursion desk (you can purchase these ahead of time online). Check out the detailed tips and suggestions for these ports at our sister site www.portreviews.com