PACK LIGHTLY! Staterooms are small! There is often hidden storage in your stateroom. Ask your room steward for a tour:)
Bring some laundry pods to use the ship's laundry facilities....or you can purchase soap, or pay for laundry service
Bring a lanyard to keep your ship key card in. You will need your key card to access your room, order drinks, get on and off the ship at every port, and to make any purchases onboard. Guest services will often punch a hole in your card, but bring a lanyard!
Bring a watch and a travel alarm clock or your phone! Clocks are hard to find on the ship.
Bring a reusable water bottle to fill in the dining room. Often you'll have to ask for a glass of water then transfer it to your bottle yourself as they won't refill water bottles
If you bring wine, remember a corkscrew
Bring a highlighter to highlight the activities that you want to do in your daily cruise activity. You can bring magnets to attach it to your stateroom wall which should be magnetic
If you are travelling with others, bring a magnetic white board and markers so you can leave each other messages about when and where to meet etc
Bring more sunscreen and first aid supplies/medication than you think you will need. They are expensive to purchase on board!
Bring an empty bag to separate your dirty laundry from your clean clothes or a collapsible laundry hamper (can buy at Dollarama)
Cruise ship rooms often have very few outlets and usb ports. Bring a multiport usb charger and an external battery charger!
An over the door storage hanger (you can buy the collapsible "drawer" type or shoe pocket hangers at Dollarama!) There is limited space in the bathroom and the closet and you can use this vertical pocket storage space to store your toiletries, clothes etc.
Check the dress code for your cruise especially for the dining room:)
Many cruise ships now have flat screen tvs with HDMI ports so you can bring a tablet with pre-downloaded Netflix with an HDMI connector and cable if you like to watch tv-cruise TVs often have limited channels...
Wrinkle release spray is useful since irons are prohibited on board
Bring a carry on bag than can double as a day pack for excursions
If you plan on hanging out at the pool a lot, you can buy beach towel clips and clothes pins so that your towel doesn't fly away. Clothes pins are also useful to hanging things to dry in your room or on your balcony
Bring a flashlight or nightlight to read and use the bathroom at night without disturbing the others in your cabin
CHECK YOUR CRUISE LINE'S LIST OF PROHIBITED ITEMS-it varies by cruise line, but generally irons, alcohol, coffee makers, candles, and drones are not permitted on most cruise lines