Go into the options and select "Cheats". 
Enter one of the following Codes.

Result                     Code 
--------------------------------------------
Zombie skin              - I FEEL LIKE SHIT  
Pirate skin              - KAIZOKU  
Knight skin              - I AM SHINY  
Zombie motocross         - RESURRECTED  
Pirate motocross         - I NEED SOME RUM  
Knight motocross         - GIVE ME A PONY  
(Crazy) Monkey skin      - BANANA LOVE  
God mode                 - KING CHRISTIAN  
All skins, bikes, tracks - COW*CRAZY
Skip Level               - HOME+END