On the forklift ride shoot all the barrels before and after the ride. There will be a parcially hidden barrel you will have to shoot to the left of the forklift. This will lead you to "egg cellent" where you shoot eggs and baby aliens.
Kronn Hunter Mode
To enter Khronn-hunter mode, ignore the aliens in Level 1 and shoot the policeman (three times) with your first three bullets.
Power-up Room
After your first progress report in Area 51, where it tells you your percentage and things like that, you will see a wall lined with windows. Shoot all the windows across the entire wall and you will be in the power up room, where you can see aliens attacking women and power-ups all over the screen.
Secret Area before Final Boss
Right before you fight the Final Boss when you are outside, you will go to an area with zombies throwing barrels [NOT GRENADES]. Shoot all the barrels that are not thrown, and you will go to a secret area filled with power ups.
Secret Bathroom
To see the alien restroom, you must shoot three blue lights that appear in the corners of the screen. At the beginning of the game, when you enter the building, shoot the first blue light in the upper left-hand corner of the screen. At the conclusion of level 1, while the screen is spinning, shoot the remaining two blue lights in the upper right-hand corner of the screen.
Secret Level
In the office building, you will go into a series of 2 or 3 doors with nameplates on all of them, shoot them all and then you will go into a secret room with power ups and an alien girl at the bottom.
Secret Level 2
On the forklift ride shoot all the barrels before and after the ride. There would be a parcially hidden barrel you would have to shoot to the left of the forklift. This would leed you to egg cellent where you shoot eggs and baby aliens.
Secret room 3
On the helicopter ride, shoot all of the boxes with flames on them. If done correctly, you will be sent into "X Marks The Spot", which is a room where you keep breaking the glass on a wall with your guns to reveal a picture of a female policeperson lying there in a very inviting position.
Hidden Level...