In addition to the item already given. I also take zip locks. One of the really big ones, a couple of gallon size, a couple of quart and a bunch of sandwhich size. Also an ice pack and a softsided cooler. If we have a shore excursion we order lunch from room service and pack it in the zip locks and pack it all in the cooler bag.
Also we decorate the door. if you get your room decorations you can just take one of the hanging thingys and put it on the door. It made it very easy to spot the right cabin.
Bungy cords for keeping the balcony door open when we wanted to have fresh air.
Outlet Strip/bar for plugging in chargers ect.
Post-it pad for leaving notes to family / friends and the steward.