One of my all-time favorites is the pumpkin that is drowning in a bag. This one was a Rock Cockerham invention and it still makes me smile. It isn't easy to create however.
I have failed at it a few times and I believe Rob took a couple of swipes at it before he got it right the first time. One technique that I used recently is to use about 3-4 bags and then to peel them off once the pumpkin is in position. I think it ends up looking fairly cool.