NES Cheats: Major League Baseball Cheats

Enter the bleachers:
Start a two-player game and allow runners to reach all bases. Have the runner on second run back to first base, and the runner on third base run back to second base. The player with the ball can enter the bleachers from the top right side of the field.

Special team selection:
Hold Down/Left + Select + A + B and press Start at the title screen. Use the "1 Player Set" and "2 Player Set" options that appear on the next screen to select a team.