Unlock All Characters:
At the main menu, press and hold L R, then press DOWN, DOWN, DOWN, DOWN.
Unlock All Parts:
At the main menu, press and hold L R, then press LEFT, LEFT, LEFT, LEFT.
Unlock All Tracks:
At the main menu, press and hold L R, then press UP, UP, UP, UP.
Unlock Everything:
At the main menu, hold L R, then press RIGHT, RIGHT, RIGHT, RIGHT to unlock everything.