There's a stage in the game known as Mushroomy Kingdom. However, there's an alternate version of the stage that will appear at random. If you want to force the game to choose the alternate version of the stage, hold down the R or L buttons (on the GameCube or Classic controllers) or the B button (on the Wii remote) while selecting Mushroomy Kingdom. After the match begins, you can release the button. If done properly, you'll find yourself not in Mushroomy Kingdom, but in an entirely different stage.

Additionally, if you want the "1-1" version of the stage (the original version), hold down X or Y on the GameCube controller to force the game to choose it.