Endurance Mode:
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.
Super Hard Mode:
At the title screen, hold the R button on Controller 1 and then press the following buttons on Controller 2. If done right, a sound will be heard and the "Super" mode will be available in the options.
Effect: Code:
Super Hard Up, Left, Down, Right, B, A, X, Y.