Invulnerability: 
     Collect  all  of  the  babies  in  the game. A "Cheat Discovered"
message will appear after the flying jetpack baby cutscene.

Secret Flea area:
     In the Lower East Quack level, go past the area where you hitch a
ride  on  the  subway train. Go to the end where you see a brown fence
and  a  little  hole  at the lower left side. When you are there, turn
left to find a Chang transport. Once you are Chang go though the hole,
and on to the boxes. Chang will start scratching himself all over. You
will  then  be  transported on his back as a flea with a heavy-machine
gun.

Land Of Moles Extra Multiplayer Level:
     In  the  first  level in New Quack City, where you pushed a model
building in front of a camera, there are some computer screens. One of
them  has  the  BIZZARE company logo on it. Shoot that one and it will
explode and unlock the secret multiplayer level "Land Of Moles."