The following are probably the coolest Halloween costumes ever.
For starters, check out the following video of a Bumblebee (Transformers) custome; it really transforms at 0.32! There are quite a few videos of DIY Transformers costumes on youtube, but this is the best I have seen so far.
The only improvements to it could be:
(i) enable the car to actually run!
(ii) [some one commented] a version that transforms one’s date into Megan Fox
And how about this mobile toilet costume? At first glance, it would seem that there are 2 guys there!
Can anything top those two? Probably things that magically transform into patio furniture cushions would do the trick.