Continue game play:
Press A, B(2), A at the game over screen after losing during one of the first three levels of the game. This also can be done on controller two at any time for a second player to enter the game.

Level select:
Quickly press Up, Down, A(2), B, Left, Right, A, B, Up, A, Down, Right(2), Left, B, Up, Left, A, Right, B, Left, Right, A, Left, Up, A, Down, A, Right, Left, A, Start at the title screen. The phrase "Stage 1" will appear next to the airplane landing screen to confirm correct code entry. Press A or B to change the starting level.

Double tank:
Position both players on a tank and press A on both controllers.

Double helicopter:
Position both players on a helicopter and press A + B on both controllers.