Be ready for theme parties and formal nights with the right costumes and attire.
Most, if not all, cruise lines have an elegant night or captain's dinner where formal attire is either encouraged or required for eating in the main dining rooms.
In addition to this, every cruise line I know of also has various theme parties, from glow parties (where guests are encouraged to wear neon) to '80s parties.
As a member of the entertainment department, I was always required to have costumes or attire to match the theme.
Even now as a guest, it's fun to check the itinerary before the trip to see which theme nights to expect.