Usually there are a couple formal nights, where everyone dresses up very nicely and photographers are everywhere taking pictures.
So you will want a couple of fancy outfits for those nights.
Also, there is a dress code for the formal dining room: no shorts, tank tops, flip flops allowed – that sort of thing.
So do bring some nice dress pants, shirts for men – not just shorts and tee shirts! Anything is fine on kids.
You will want comfortable shoes to walk around the ship and around the ports of call. And of course if your cruise is to a colder/cooler climate, you will want a rain jacket and layers.