This is one of my favorites...
The "Nosebleed Whirlibird High Dive"
In the Noki Bay stage, get to the top of the tallest cliff. It's the one just to the left of the starting area, with the tree and yellow bird on top. Do a triple jump off the cliff, toward the main area of the stage. As you begin to fall, hold down the R button to spray water.
It doesn't serve any practical purpose, but it's fun and it looks cool. :) Bonus challenge: Try to bounce off the floating springboard when you land.
Post your own stunts if you want. :)