this might be silly but... on our most recent cruise i got a folding laundry basket (courtesy of CM's packing list) it was great for helping keep the room tidy. Previously, clothes got thrown inthe corner. Now they go in the basket and it is much more tidy. Other must have that I actually for got was sunscreen
I was so worried about other stuff I forgot it and had to purchase it at the airport before we got our luggage and on to the ship. My DH has yet to let me forget that one
One other must have that doesn't get mentioned a whole lot is a mini first aid kit - band aids, neosporin, alchol wipes and calamine lotion.
If you google Cruise Packing list you get all kinds of things.