Here's some comedic suggestions:

#1: Go grab a fake gun, splash some water over your face, begin panting, open the door, and immediately say "Can't you see I'm busy right now?!"

#2: Rush to the closed door, and begin barking loudly, scraping at the door, and rummaging around the letter box. (I've done this; it works!)

#3: Open the door, but act as though you have an invisible friend right next to you; now talk to this invisible none-existent friend, and act all offended when they don't address themselves to him or her, shortly after, close the door in disgust.

#4: The moment you hear the door go, and you have seen from out the window, that it's a cold caller, start to loudly - but not OTT - talk about someone you recently put out of commission, and try to give some details about where you buried the body. Continue talking about it as you go to the door, and then open the door and act all innocent, but mention that you're a bit busy cleaning up.

#5: Answer the door whilst facing backwards, telling them that if you look into their eyes, they will turn into frogs, so it's for their benefit - see how long they last.

Those were just randoms from my brain; I'm sure there's many, many more. :P Have fun!