Hint: 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.

Hint: Secret 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