Order a bunch of Peanut Butter and Jelly Sandwiches from room service the night before an excursion. They come to you in the form of Smuckers Uncrustables, so they are super easy to throw in a bag and take with you. It'll save you from buying snacks while you're in port, which, depending on where you are, can be quite pricey!!!
Also, another great item to pack....a very small sized room spray. With multiple people sharing a small space, it really comes in handy for the bathroom. Bath and Body Works and Yankee Candle Company both carry small little spray cans that you can easily pack.
If you plan on doing some duty free shopping while cruising, make sure you price out some of the items that you are interested in BEFORE you sail. My husband and I bought a bottle of a "rare" tequila in Mexico, thinking we got a great deal on something unique, only to find the same item for sale at home at Sam's Club for less money (AND we wouldn't have had to lug it home from Mexico, if we had known!) Same thing goes for perfume, cologne and jewelry! Make sure you're getting a good deal by being knowlegable before you sail!
I will also give my support to the idea of packing lots of various sized ziplock bags. First of all, I always put my toiletries into them before I pack them. That way, if something were to open in my luggage, it isn't going to ruin any clothes, or make a huge mess. I also use them to portion out candy (which I bring with me from home) for visits to the Buena Vista Movie Theater, the Walt Disney Theater or the Ariel View Screen. They can be used for dirty laundry, for wet bathing suits after excursions, and also as a make-shift cooler. When you're getting off the ship, pack the bag with ice and leave room to throw in some bottled water or snacks. There really are TONS of uses for Ziplock Bags! If anyone else has some good ideas, throw them out there!