Legend4life95,
Congrats, you are going to have a great time. I've been on 2 crusies lately, one Western itenerary on Carnival, and one Eastern on RCI.
First, I would highly reccomend you check out
www.crusiecritic.com. You can do hours of research there on various cruise lines and destinations. Great chat boards too.
On my Western trip, I had the same stops as yours other than we went to Isla Roatan instead of Costa Maya. Cozumel, lots of fun things to do right where you doc. There is no tendering required here, you can just walk off the ship. Belize, pick a good excursion here. Very beautiful country, but also a third world counrty. Lots of poverty here. In Grand Cayman, you absoultely have to do one thing if nothing else on your entire trip. Sign up for an excursion to go swim with the stingrays. Out of both crusies, that was the most fun we had by far.
One last thing, is your sail date. October is hit or miss. The weather should be perfect. The drawback, is that you are right in the middle of hurricane season. On our first cruise we had originally had an Eastern cruise. On the day of, they moved it to the Western to avoid a hurricane.