Hold down X + B + Z + L + R, then turn the game on.
Invincibility
At the Start Screen, hold A + C + X and press Start.
Music Test and Outtakes
Collect all the Rainbow Amulets, then go to the Rainbow Junction shop, and examine the empty space on the far right of the counter. You'll find an invincible item that requires you to have all the Rainbow Amulets. If you buy it, you'll unlock "Mokona's Home" and "Outtakes" in the main menu after beating the game once.