I'm curious, will there be anything special on the Carnival Holiday during Christmas in the way of decorations? My kids have asked if they will have a tree up! I honestly didn't know what the cruise lines do during holidays since obviously not everyone celebrates. I'd love to hear from others that have traveled Christmas week. Will businesses be open in port (Cozumel and Playa del Carmen) on Christmas eve and day?
From my own experience (which does not include Carnival during Christmas) and what others have related, most cruise lines have lavish Christmas decorations. I think it's safe to tell the kids that there WILL be a tree. More likely, LOTS of them.
I wouldn't even hazard a guess on shops being open. Why not try a GOOGLE.COM search on the ports to see what you can find?
You can be assured that hotel restaurants, etc. will be open.