CruiseStar
The stuff people forget every time
A non-surge-protected power strip for the limited outlets in most cabins (surge protectors are usually banned), a small day bag for excursions, motion sickness remedies even if you've never needed them before, a lanyard for your room key, and a portable phone charger for port days are the items sailors most often wish they'd packed. Cabins already provide bath towels, pool towels, and basic shampoo, soap, and conditioner, so bringing your own toiletries is a preference rather than a necessity.
Formal night doesn't require what you think
A collared shirt and slacks for men, a dress or nice separates for women, covers the overwhelming majority of formal night dress codes on lines that still hold them, a jacket is a nice touch but usually optional. Formal nights have gotten noticeably more relaxed industry-wide, so it's worth checking your specific line's current policy rather than over-packing for a strict standard that may not even apply anymore.
Climate and logistics
Some itineraries genuinely span multiple climates in one week, Alaska into a warmer port, or a Caribbean cruise with one unpredictable sea day, so light layers beat committing to a single weather assumption. A passport is strongly recommended even for closed-loop cruises that technically only require a birth certificate and photo ID, and it's worth keeping your cruise documents and any travel insurance paperwork somewhere accessible rather than buried in checked luggage.
What goes in the carry-on
Medication, a swimsuit, a full change of clothes, and your travel documents belong in whatever bag you carry on, since checked luggage can take several hours to reach your cabin on embarkation day, and you'll likely want to start enjoying the ship well before then. Most lines allow a reasonable amount of sealed snacks and typically one bottle of wine or champagne per adult at boarding, though rules on cans of soda or cases of water vary enough by line that it's worth checking in advance. A little cash is also worth having on hand for port-day taxis or small vendors who don't take cards, even though nearly everything onboard runs through your ship account.