On the main menu, press the following to unlock the different bonuses:
Extra skin 1:
SELECT, LEFT, RIGHT, LEFT, RIGHT, LEFT, RIGHT, LEFT, RIGHT, LEFT, RIGHT, LEFT, RIGHT.
Extra skin 2:
SELECT, LEFT, LEFT, RIGHT, RIGHT, LEFT, LEFT, RIGHT, RIGHT, LEFT, LEFT, RIGHT, RIGHT.
Extra skin 3:
SELECT, LEFT, LEFT, LEFT, RIGHT, RIGHT, RIGHT, LEFT, LEFT, LEFT, RIGHT, RIGHT, RIGHT.
Extra skin 4:
SELECT, LEFT, LEFT, LEFT, LEFT, RIGHT, RIGHT, RIGHT, RIGHT, LEFT, LEFT, LEFT, LEFT.
Extra music track:
SELECT, UP, DOWN, LEFT, RIGHT, UP, DOWN, LEFT, RIGHT, UP, DOWN, LEFT, RIGHT.