Field of Corn:
At the game option screen highlight Credits and press Square, Circle, Square, Circle, Triangle. After it returns you to the previous menu you will know the code worked if the stadium name has a green color to it

Wander Around:
Go to the right dugout in the King Dome, then go to the closest or farthest side of the dugout. You should be able to run around outside of the stadium.

All-Star players:
Highlight the designated hitter selection on the options menu and press Square, Circle, Square, Circle. Then, the AL and NL All-Star players will be available after exiting the menu.