Unlimited ammunition
Hold [Option] and select "New Game". Release the key after the weapon level increases to 999999 in the tool box.

Nuclear difficulty level
Hold [Option] and select "Difficulty Level".

Nuclear missiles
Locate a plane and shoot it down. Completely destroy the plane just before it reaches the ground. A prompt offering nuclear missiles will appear. To fire a nuclear missile, hold [Option] while firing a missile.

UFO
Enable the "Nuclear missiles" code, then fire all missiles as nuclear. Obtain another set of missiles, and fire them as nuclear. A UFO will appear while the second set of missiles are being fired.

Super shield
Select the machine gun and shoot the parachutist twice. Then, shoot him once more just before he reaches the ground.

Earthquake
Select the laser and fire the weapon a few times in the lower middle section of the field. When the correct location is hit, an earthquake will that destroy everything on the ground.