Monster Allergies
Each monster is allergic to something. If a monster eats the thing it's allergic to it will sneeze a building destroying sneeze.
George allergic to dogs
Lizzy allergic to birds
Ralph allergic to cats

Ride The Tanks
Jump on tanks to ride them, then you can control how the tank moves. After a while the tank will explode and shoot you into the air.

Ride The UFOs Jump on UFOs to ride them, then you can control it. The UFO will eventually disappear.

Hidden Cities Cheat
To access the hidden cities, rapidly hit the button for the monster you chose on the screen that shows the name of the city you're about to enter until you get a message saying "Secret Code Accepted"
If George Hit Jump
If Lizzie Hit Punch
If Ralph Hit Kick
This is a list of the cities you will go to depending on which city you select:

City Hidden City

San Diego Caleb's City

Kodiac unknown

Fairbanks Underworld

Washington DC unknown

Atlanta Suburbia

Nashville unknown

Louisville Warehouse

Moab unknown

Phoenix Area 69

London unknown

Kiev unknown

Casablanca unknown

Rio de Janeiro unknown

Character Color Change
Go to the character select screen and press up on the control stick when selecting your character to choose the color of the character.

Infinite Lives Cheat
At the City Screen where it shows the name of the city and a hint, press B once. You will get a message saying Secret Code Accepted, then when you die, press Start and you will continue, and you can keep doing this as long as you want.

Access The Hidden Character V.E.R.N
To play as V.E.R.N. find a Scum Lab facility. Eat the toxic waste barrels and you will turn into V.E.R.N. for the rest of the stage.