Triple Play '97

Cornfield: from the Select Stadium screen, press up, down,
 right, up, down, left, up.
Monster Home Run: hold down the 1, 2, and 7 number keys while swinging.
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.
The following can be used in any combination, 
i.e. depress the 1,2 and 7 keys for a home run, etc.
Bunt: Depress the 1 and 6 keys simultaneously.
Ground Ball: Depress the 1 and 4 keys simultaneously.
Line Drive: Depress the 1 and 3 keys simultaneously.
Long Fly Ball: Depress the 1 and 2 keys simultaneously.
Pop Up: Depress the 1 and 5 keys simultaneously.