Enter the sound options, highlight "Mono", then hold X and press Right, Up, Right, Up, L1, R2, L1, R2.
All mini-games
At the title screen, hold R1 and begin playing a mini-game. After completing the mini-game, hold X + Triangle at the summary screen to access all other mini-games.
Bio Rhythm
Select the "End" command and highlight the first option. Then, hold X, press Right(4), L1, Up, Start, Right.
Inner mode
When the SD character begins moving while the game loads, hold X and press Right, Up, Left, Up, Right, Down, Left, Up, Triangle.