First the Kale helmet: the ultimate, last minute, low-cost costume (somewhat themeless but hey it's in season). All you need is a well-stocked grocer - if not just use another vegetable, preferrably one that doesn't wilt.
Other costumes with shortish half-life include the Office space costume and the balloon costume I once tried for a "Dot"-themed party. However, these quickly come undone as you can see below. (All you need is tape, string and a bunch of balloons. I recommended postponing assembly till you're right outside your destination.)
My friend Chris has been more elaborate dressing up as white noise (in a cardboard frame on grey pants?), notice though how people have been plucking away at his friend's Office space costume.
Cardboard in general gets you far: The Yemen package outfit below is pretty bold given recent events but so low effort and effective that it merits attention.
In the realm of the freakish - check out the cardboard deer on the F train below. Elaborate but again not hard to make: duct tape, glue, stapler?
Re the versatility of cardboard - see this twitterbot - and my own dionysian mask for Simon Goldin's MFA graduation:
Headgear in general is ultra-low effort: Card crown and necklace-cape for casino theme party (deck of cards, tape, paper clips, rubber bands).
Below a bird's-nest-in-your-hair outfit for a "do whatever you want with your hair" -theme party: (google image search, color printer, cardboard, glue, eggs, hair pins. Empty the eggs though with a needle to spare people the temptation...:)
Last, speaking of animals - the antithesis of a last minute fix - this doggy showed up at the Halloween dog parade in Tompkins Sq. Park.