B.C.
My wife and I just got off Carnival this morning. WE HAD LOTS AND LOTS OF FUN. This was our 15th cruise and we've been on Carnival 3 or 4 times.
I always bypass the shops recommended by the cruise lines, except if I was going to buy precious stones/gems. I can get a much better deal from the locals and I feel I am helping a family make a living rather than making the cruise lines richer.
My wife and I enjoyed Tulum and felt it was worth while. We had a good guide and he made it interesting, When on a guided tour sponcered by the cruise line, 10% is a HUGE tip. Why, because the tour price is severly inflated by the cruise lines.
I enjoyed the cave tubing and had a wonderful time. We stopped enroute to the tubing drop off point and bought fresh sliced pineapple from the locals. They let it fully ripen before they harvest it and it is by far sweeter than anything I've ever eaten out of any pineapple I've bought in any grocery store in the states. They put it in a baggie with some of the pineapple juice. My wife and I and two other couples we knew that bought some that way never had any sickness or bowel problems.
As a matter of choice I never use any locally made ice in my drinks. I use cruise ship water for drinking. I bring along an empty (Airport security issue) a wide mouth juice bottle and fill it up on the ship with water and ice and take it with us on the tours. Carnival charges $4 for the same water.
Get late dinner seating if possible. That gives you time to get back from your tours with out having to rush to dinner.
If you buy anything from the locals, which is fun to do, ALWAYS bargain with them and NEVER pay full price. They sneer at us Americans and call us fools, and have absolutley no respect for us as a people if we pay their asking price. A good rule of thumb is to start at no more than 50% of the price they are asking for. I almost always get what ever they are selling for 30% to 60% of their initial offer. And don't be afraid to say "No." On more than one occasion I have had a vendor tell me I did a "good barganing".
In Belize, there is a high wall around the "recommended" shopping area. Outside the wall, prices are much cheaper and the locals are far more friendly and helpful. There are two or three vendors selling fresh coconuts and straws outside the wall. The coconuts are $0.50. The vendor puts the coconut on a spike and peals off the outer skin. He then uses a machette to cut the tip off. He gives you a straw and you enjoy. When you are done with the coconut juice, you bring back the coconut to him and he cuts it open and cuts the meat out. You get to enjoy the meat too. Its far, far better and fresher than anything you can possibly get in the states. The taste is noticably better too! Take a video of him doing it. It's unique and facinating to watch when you get back home.
If you have any other questions, e-mail me.
Good luck to you and yours.