Minecraft wiki entry for Building_a_cruise_ship, associated text: Cabins for persons to live on the cruise ship. You may have different classes for cabins - ranging from ones with nothing but a bunk bed to suites with balconies and king-size beds. More expensive cabins tend to be placed on upper decks.
Ice Hockey on large cruise ships
Spleef arenas
Nightclubs
Casino
Restaurants
Shops (taxfree or so): A cruise ship doubles as a mall, as it has so many different shops. Make sure to have many shops for exotic and luxury goods.
Playrooms
Smoking rooms
Soda vending machines
Jukeboxes to play music (remember to put a music disc inside the jukebox or the jukebox won't play any music)
Info desk
Swimming pools
Waterslide
Engine Room: A ship cannot function without an engine. Engine rooms are huge, with tons of complex machinery. Building an engine room may be tricky, as many have never seen one.
Car Deck
Bridge: Any ship needs some form of a command. Build a glorious, wide bridge from which the Captain can lead the vessel.