From http://hem.bredband.net/b257182/Cows&Easter.htm:

Attack From Mars, Bally 1995

Sometimes when you shoot the Big-O-Beam, a cow is displayed and a female voice says: "Would you look at the size of that cow?!" and a moo is heard.
Sometimes when you shoot the Tractor Beam, a cow is beamed up. (View animation).DOHO is also seen in this animation.
Whenever one of these animations appears, hit the Launch button repeatedly for lots of mooing sounds. Do this for both of the ramps and then the next Video mode will be Cow Attack Video mode, where the
saucers has been replaced with mooing cows.

At very rare occasions one of the saucers in Saucer Attack Video mode will be replaced with the number 3. The smart bomb (see below) has no effect on the "3" and nothing happens when it reaches the ground, it just disappears below the horizon. The programmers added this easter egg as a "homage" to a bug in the ST:TNG video mode which replaces the items in the tunnels with a "3".

Shoot the same Martian during Martian Multiball five times consecutively. Skull from No Fear appears saying "Again!"

Smart bomb: Press Launch button during Video mode to clear screen (works only once).

Tournament mode: Insert credits and hold both flipperbuttons for a couple of seconds before you press Start. The display will confirm that the next game will be in Tournament mode (Stroke of Luck award
will always be 50M).

"Eat at Eddy´s" can be seen on the backglass (designer Brian Eddy).

One of the default names in the Buy-In Score list is MOO.

The woman on the backglass is the secretary who worked in the reception at the WMS factory.

In the DMD animation for the Atomic Blaster Hurry Up there is a road sign passing by. This sign reads DOHO.

During Attract mode, press the following sequence:
B, 18L, 1R, 3L, 1R, 6L, 1R. That spells RCF and shows the credits for Attack from Mars.

AFM 1.13:
2L + 1R + 18L + 1R + 5L + 1R ; Moo sound, replaces video mode (ball in lane)
12L + 1R + 6L + 1R + 19L + 1R
1L + 1R + 2L + 1R + 3L + 1R
6L + 1R + 13L + 1R + 3L + 1R ; "18" (attract mode)

Enter the tag MOO in the high score list and a nice "moo" will be heard.