The cabins on a ship aren't like the typical rooms you'd find in a home or hotel. Instead, they are made of metal, like much of the rest of the ship. So, many who take cruises regularly will use this little hack to get some more storage space while they're out on the open ocean. Just bring a magnet with you to hang things like notes, menus, and even your room key.
You'd be surprised how handy it can be to be able to leave a note for others staying in your cabin, especially considering you probably won't be able to use your phone on the ship.