At the player select screen, hold the R button then press A, Left, Y, Right, X, Down, B, Up on Controller 2. You'll play a special version of the game in which you face all of the bosses.
Play as Rabio
To get the character Rabio go to the character select screen and hold Right Shift. While holding this use Controller Two to press Left, Down, Right, X, Y, A, B, Left, Down, Right, Down, X, Y, A, B.