this isn't a halloween costume, but I figured this was the most appropriate board, nonetheless...
My son is turning 4 (in a week!!!) and wanted to have a superhero themed party this year. He has asked that his friends all come dressed as their favorite superheroes, too.
We have been working our butts off to DIY as much as we can to prep for next weekend's craziness. Up until Thursday night, he has wanted to dress as Spiderman, and I was at a loss as to how to get a costume done in time with everything else, or where to buy one this time of year without shopping online. Well, Thursday when we were talking about it, I had suggested he wear his Spiderman jammies, but then he says he never wants to be Spiderman. He wants to be batman.
Glad I never started making a Spiderman costume

Anyways, I sat down and tried to think this out, because he also has Batman jammies, but wouldn't wear those either.
We went to pre-order balloons, and they had a plastic batman mask for $5, so I bought that thinking I could throw a towel on him dressed in black and call it done.
Well, then it hit me! Batman has MANY different costumes over the years, and one of my personal favorites was the gray costume. This past halloween, DS was a gargoyle
http://www.craftster.org/forum/index.php?topic=394104.0, and I made him a gray jumpsuit with a removable tail, wings, etc.
I know I have a good 4m of black jersey sitting unused in my sewing room, and I HAVE to have something yellow, somewhere in this house that I could cut up.
Upstairs, I pulled out a dingy, stained yellow t-shirt, the black jersey, last years costume & some felt. I cut a tube from the bottom of the shirt to make a really quick (utility) belt. I skipped the utility part, and just sewed a tube, then turned it rightside out & topstitched it to keep it flat. I cut out a large rectangle of black jersey, rounded the top off, cut a oval for his head, and split the front for ties. I made jagged cuts like Batman's cape. I then used the cut out oval as the base for his bat symbol, and made a matching oval out of yellow felt (sturdier than the jersey) Cut out a bat symbol and stitched the black in behind. I hot glued another piece of felt on the back, then some safety pins with small strips of felt & more hot glue.
He was skeptical of everything I was doing, and kept a VERY close eye, so Mommy didn't mess anything up, but when I had it all done (about 45 minutes from start to finish), I had him put it on & test it out. HE LOVES IT!
He's running around the house in full gear, and won't take it off. He wants to wear it to bed...

We decided to skip the gloves/bracers as he will be busy playing games & running around with his friends. He also didn't like the idea of "underwear on top of my pants - that's silly Mommy!"

Comments/Criticisms?
I know there's lots of room for improvement, but this was such a quick, simple solution!