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.