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 multi-player level
In the first level in New Quack City, if you remember pushing a model building to block a camera, there is a computer in the computer room that is destructable. Shoot it until it explodes and you will unlock a secret multi-player level. Note: Your green target will turn red on the described computer . The target will turn red if you can shoot something and get a reaction -- keep that in mind when exploring.
Matrix-style scene
In the Lower East Quack level, shoot three baskets in a row without missing on the basketball court. Press Start to start the view.
Dynamite Room Help
This isn't a cheat, but it really is good. In the dynamite blaster part in Beaver Power, there's a room where if you shoot, you die. Use the bomb launcher, and attract the gators to the entrance, where if you shoot, you won't die. Then just kill them one by one !
Invulnerability
Collect all of the babies in the game. A "Cheat Discovered" message will appear after the flying jetpack baby cutscene.
Time Slice Camera
Beat the basketball mini-game at the Lower East Quack level to unlock the Time Slice Camera.