Eveningwear - some people really dress up . especially Formal Nights . Evening dresses , sequins , jumpsuits is a good shout too ( I wore a Phase Eight jumpsuit last time)
Others will be very casual .Jeans , casual trousers etc.
One of our trips had a White&Gold Night (I didn't have this bit I cannot remember how many people did .)
I bought some dresses from eBay , BNWT or VGC .
Monsoon, Phase Eight , Roman
M&S had great deals on their Christmas party wear , I got skirts and tops sequinned .
For day time take things that will match with loads of others , I dry and make each top or bottoms go with three other things . Or dresses ( WhiteStuff or Seasalt )
Comfy shoes 
Don't forget you can use the laundry either Self Service or have them laundered ( added £ , the self service is a few pounds )
M&S linen mix trousers ?
The ship will be air conditioned inside , the restaurants and public areas
Your sea days are mosey about and try things out days ( or just relax and read )
Enjoy .
Where are you going to ?