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