To play in Mystery stadium, from the Select Stadium screen, press the arrow keys (or directional buttons on a gamepad) in this order: right, left, up, left, down, right, left.

To play in the Cornfield, from the Select Stadium screen, press up, down, right, up, down, left, up.

To hit a monster home run, hold down the 1, 2, and 7 number keys while swinging.