WARNING! These are not the most pleasant things to think about and I may be perceived as a Germ Nut..But lessons learned are not forgotten.
WATER bring all you can, you know something is up when you are having a problem and then you see the crew buying water in port.
Imodium..in case you forget the water!
Small can of Lysol
Individually wrapped anti-bacterial wipes so when you walk down all the steps with handrails 2000 other people have used or went to restroom on Lidi Deck..ya might want to try one before you pick your Cheeseburger up.
Cheap Snorkel Tube..Even if you rent or take an excursion that includes equipment..I am not big on putting something in my mouth that someone else just had in theirs..LOL
Bullfrog Sunscreen! .. If you are going to be in the water! Even if your brand says waterproof and sweatproof the salt water will disintegrate it fast. It is made for water and they have some for kids now.
Mini Igloo Cooler..Is small enough to fit in suitcase with 6 beers and always put another 12 pack in the other suitcase.
CASH..so you can give the Steward a little extra tip when they see and say "I bring Ice"
Patience, and a good Attitude.