At the title screen press Down, A, Up, B, A, Left, A, Right, Down ("DAUBALARD"). The short sound will denote correct code entry.
Play a Secret Game
At the title screen press Left, A, Up, Down, A, C, Right, A, B ("LAUDACRAB"). The short sound will denote correct code entry. You will get to some variety of Snake game.
Unlimited Jump
At the title screen press C, Right, Up, Down, A, C, Right, Down ("CRUDACRUD"). The short sound will denote correct code entry. You will be able to jump as high as you want and from any point, even in the air.
Unlock all levels
At the title screen press Right, Up, Down, Down, Left, Right, A, Up, C. The short sound will denote correct code entry.