Bonus Stadiums:
Press Left, Left, Up, O, O, Triangle at the main menu to access extra stadiums.

Additional Features:
Press Left, Up, Left, Up, Left, Up, Left, X at the cheat screen to unlock some new features.

Bird Mode:
Players look like bird people when you press Up, Triangle, Left, Square, Right, O, X, X at the cheat
screen while the game is paused.