Double Dragon Advance
  
Game Boy Advance
  

Expert mode:


  At the Options screen, hold Select and press Up, Up, Down, Down, Left, Right,
    Left, Right.

10 credits :


At the Options screen, hold Select and press L, R, Down, L, R Down, L, R, Right.
Change music:


At the Options screen, hold Select and press R, L, R, L.
  

Thanks to Revolution reader Cheesy Poof!