How long is it and will you have any formal nights on board?
Check what plug sockets will be in your cabin. You can usually find this out just by googling. You might need to take an adapter.
A small or medium backpack or tote bag can be useful for carrying books, sunscreen, water bottle, purse etc.. around the ship and in port.
If you're a light sleeper or have a room in a noisy area of the ship (low decks near the anchor, engine or thrusters; directly under the pool deck or buffet), you should pack a pair of earplugs.