Ziplock baggies. They are great to have to put any shells or sand you may collect. Also it is a good idea to put bottles liquid stuff in baggies before putting them in your luggage that way they don't break open in your luggage and get all over your clothes.
I also put a collapsable duffel bag in my luggage. I know you can only check one bag for free now but I am still packing the duffel bag and will pay for it if necessary. But anyways, I put my dirty laundry in the duffel bag then have more room in my luggage to wrap souveneirs and such and don't have to worry about it.
I also heard to pack a highlighter to mark your capers and a pack of post it notes so you can leave notes for each other on the door. I plan on doing that.
I also agree with the alarm clock especially if you have an inside cabin. You have no idea what time it is always dark.