Once we brought extra soda on a group cruise in case others wanted some diet soda and wound up not drinking it all. Good advice to not overdo. Its not like you are going to bring a can to the dining room that you lugged in your suitcase. So dont bring for dinner and maybe a few lunches, you will be ashore during some lunches or maybe have a lunch or two in the dining room.
Last time we brought about 7 diet cokes for my sister, I dont drink them, so that was for one person. I wouldnt bring canned juice. If you are on a Conquest class ship you can get extra juice at breakfast and put it in the refrig for later.
I would bring hair conditioner. Carnival only provides shampoo but not conditioner. I have long hair and cant do without it.
We bring an extra fold up bag that is carry on size and fold it into a suitcase to bring home all the stuff we shop for in ports, we love to shop.