Right after the miniboss in Bubbly Clouds, Kirby will fall down a waterfall. Beside it there is a mint - if you swallow the mint while falling in the waterfall you will blend into the background. If you touch the water the glitch will be canceled. Also keep in mind that you aren't invincible, - you're just like normal except you are inside the background. The effect wears off once you move to the next room.

Any time during the game pause the screen and wait a few seconds. Kirby will dance around on the paused screen.

On the title screen....