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.