If you're more "Meh, who can be bothered?" than "Awesome! Candy me up!" then the best possible costume solution is the DIY Minecraft Creeper head. It's basically a box with some paint on it. Easy. Lazy, even.
If you have nothing on today, then consider making a Reshiram. You don't know what they are? Watch the video and wonder where some people get their spare time from.
Are you handy with the eyeliner? And you're a cute girl? Then manga is a great option. Just lay on the MAC colours nice and thick, then throw on a colourful wig. Easy. Now all you need to is to throw some cute manga girl poses and you're set.
If these don't grab you, consider strapping on a couple of huge plasma TVs and voila, you're an instant iPhone.
Happy trick or treating.