Mistress Za Warudo
The gift of friendship.

But no, I got nothing.
They might never even show it and Luffy will just leave it behind.
Make up some excuse like it was nothing special and just smile it off as they embark on another adventure. I unno.
Lawl at the dress idea though.
Maybe... But considering the lengths they had to go through to get to where they are, I doubt they'd leave it behind. Of course, knowing Oda, despite the the long journey, it really would be something silly which would cause everyone but Luffy to freak out.
Future note: The Japanese word for dress is 「ワンピ-ス」 "WAN PIISU" as in "One-Piece Dress"