Hidden Level
Using George, punch the sign on the first level that changes pictures while it is on a western scene. When you get to San Diego, at the screen where it says what level you're on, rapidly press the punch button. A little box should then say "Code Correct." The hidden level's name will replace Carson City.

Change Character Colors
At the Character Select screen, press either Up or Down on the d-pad or the control stick.

Play as V.E.R.N.
Eat the toxic waste barrels near a Scum Lab facility and you will be transformed into V.E.R.N. (Violent Enraged Radioactive Nemisis) for the rest of the stage.

Hidden Cities
On the screen that shows your next city, rapidly punch the button corrosponding to your character:
George Jump
Lizzie Punch
Ralph Kick

Allergies
When a monster eats something it is allergic to, it will sneeze and knock down the building is on. Here are their allergies:
George Dogs
Lizzie Birds
Ralph Cats

Ride Tanks
If you jump on a tank, you can ride it control the direction it moves. After a short while, however, the tank will explode and thrust you into the air.

Ride UFOs
If you jump on a UFO, you can control which direction it moves. It will still fire, so you will be able to destroy building with it. The UFO will disappear after a while.