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