Headquarters

In the middle of wave 4, shoot two pictures on the desk.
Then you will approach two doors each with a name plate.
Shoot nameplates to go to headquarters.

Secret Room

In the hangar in wave 2, shoot all 15 windows

Puzzle Room: When the game starts, shoot the first 10 windows on the left. This takes you to a room where you must shoot a puzzle. Shoot all the pieces and a S.T.A.A.R. Team girl appears.

Tanktop Girls: In 2-player mode, shoot the first 15 windows as soon as you enter the warehouse. This takes you to a room with tanktop girls.

Baby Aliens: When you enter the building from the helecopter, shoot the "EXIT" sign when you see a purple alien and then shoot the first three lights. This gives you the opportunity to shoot baby aliens before they form.