The cabin matters more than the ship
Mid-ship, mid-deck is the steadiest ride and the shortest walk to everything. Interior cabins are dramatically cheaper and you sleep beautifully in them, but if you are prone to seasickness, pay the difference for a window low and central.
Know what the fare does not include
Gratuities, specialty dining, drinks, wifi and shore excursions are the four places the bill grows. Decide up front which ones matter to you and buy the package before sailing, where it is consistently cheaper than onboard.
Book port days independently
Ship excursions are convenient and marked up heavily. For ports where the town is walkable from the pier, skip the tour entirely. For ports where it is not, a reputable local operator usually costs half.
Pack for the ship, not the destination
A power strip without surge protection, a magnetic hook for the steel walls, motion bands, and one outfit smarter than you think you need. Everything else you can buy or live without.