**Play as the Doot-Doot Sisters**
You must first get every shine in the game except for the first one on Bianca Hills. You then find all the locations of the Doot-Doot Sisters in alphabetical order of quest names. Each time you find them, you must stand between the Doot-Doot group and the audience and do a spinning spray, showering all present. If you got everybody at once, the Piantas in the group will all yell at you.
After spraying the Doot-Doot group for the last time, go to the Bianca Hills' first shine. Instead of the muddy piranha plant, you'll see the Doot-Doot sisters and the guitarist standing at the hill with their arms crossed. Talk to them and they'll chew you out for a few hours, until the nearby chuckster comes, annoyed with all the racket, and throws you back to the Mushroom Kingdom (or so we assume.) The Piantas then have a short conversation about who'll finish cleaning up the island, and will finally nominate none other than the Super Doot-Doot Sisters.
The game will then start over, allowing you to play the entire thing with the Doot-Doot Sisters.
Go Moon!