I guess this topic will apply to all cruise lines not just Carnival.
I always like to take a photocopy of the main page in my passport.
If for some reason my passport went missing then this would help prove who I am.
There are many things to pack most people do not think of. Plastic trash bag to put dirty clothes in, Q-tips, a band-aid or 2, for the coffee drinkers a bigger mug (me one with a top) just to name a few.
1.save room in your luggage for souvenirs, you will probably be coming home with more stuff than you left with
2. think about what shoes you will need...
One thing we find very helpful is that since the bathroom vanity area is so small, we hang one of those (note sure of proper name) divided cosmetic bags that comes on a hanger on the bathroom door. That way you can put sunscreen, hair spray, etc. in the compartments and keep your vanity and desk neater. Another is a lanyard for your sign and sail card. While the ship will punch it for you, I like the lanyard because you can keep other small items in it as well. One more is a dry bag for excursions.