Mugiwara Theatre - Orchestra of the Sea

Orchestra of the Sea is the first and only omake so far that appears in the actual One Piece volumes. It is published in volume 32. It is in chapter 304, as an omake as well as a color spread. So far, this is the only omake that ever had color in the manga.

Story
The Straw Hat Pirates decide to play beautiful music in order to lure out fish so that they can eat something for dinner. Zoro comments that fishing with Chopper as bait would be much quicker, much to Chopper's shock. When they start playing the music, the legendary Rainbow Moray Eel brothers appear. After the Straw hats comment on the rare sight, they get ready to catch them for dinner, which enrages Nami. Luffy seems to have gotten punched in the end by Nami, judging at the big bump on his head.

Instruments
Luffy: Cymbals

Zoro: African drumsets

Sanji: Trombone

Usopp: Banjo

Nami: Grand Piano

Chopper: Maracas

Robin: Harp