Bad and Good Food  -  When a World Tour is over and the name of an American city appears on screen, hold the joystick Down, then for George press Jump 3 times. For Lizzy press Punch 3 times. For Ralph press Kick 3 times. This will change all bad food to good food during the next stage. 

Beware of Rubber Duckies  -  If you find a rubber ducky, don't eat it. Doing so will call the fighter planes early. 

Change Character Colors  -  To change the colors of George, Lizzie or Ralph go to the characters select and press UP or DOWN on the controller stick or the d-pad. 

Double Points  -  You can double the points you get in certain cities if you press the character button on the screen that introduces the next city. The buttons are: 

George Jump
Lizzie Punch
Ralph Kick

These are the cities that are affected by this. 

Casablanca
Kiev
Kodiac
London
Moab
Nashville
Rio de Janerio
Washington DC

You also get these affects if you pick up a calculator in any city. 

Hidden Cities  -  To access the hidden stages below, at the city screen, rapidly press the button indicated for the character you're playing: Punch for Lizzie, Kick for Ralph, and Jump for George. If you pressed the button quickly enough, you'll warp to the following stages.

Atlanta - Suburbia 
Casablanca - Palace 
Fairbanks - Underworld 
Kiev - Exchange 
Kodiak - Snow City 
London - Big Ben 
Louisville - Scumlabs Bioweapons 
Moab - Game Refuge 
Nashville - Downtown 
Phoenix - Area 69 
Rio de Janeiro - Sunset City 
San Diego - Caleb's City 
Washington D.C. - The Capitol 

Level Select  -  At the character selection screen hold L and all four C- buttons. Keep holding them until you hear a noise, then start the game. When the screen that shows the next city is displayed, use UP and DOWN to change your country or LEFT and RIGHT to change your city. Even the bonus levels are included. 

Play as T-Rex  -  Beat all the levels, then start a new game. Highlight Ralph on the character select screen, hold Z and press L, R, L, R. If you hear people scream, the code was entered correctly. 

Play as V.E.R.N  -  To play as V.E.R.N. (Violent Enraged Radioactive Nemisis) search for a Scum Lab facility. Eat the toxic waste barrels and you will be transformed into V.E.R.N. for the rest of the stage. V.E.R.N can fly, throws fireballs and has super strength. 

Tank Ride and Plane Ride  -  Jump on tanks to get a free ride. You can control the way the tank is moving. The tank will then shoot elsewhere. In a short while, the tank will explode and you will be shot into the air. You can also jump onto the low flying planes and use them to attack the buildings and other players.