We leave costumes to the very last minute. It is helpful that we have friends to borrow from and a healthy "dress-up" clothes stash, but really, I have no excuses other than Halloween is very low on my to-do list.
Are you stuck? One of the best last minute costumes was a replica of the Charlie Brown Ghost costume my friend Stephanie made for her son. Sam ended up wearing it two years in a row and now the little guys run around in it. I used quilt batting and a bit of nylon mesh (an old stocking) for the eyes and I was done! The quilt batting is warm and a bit stiff, so it doesn't get caught up in places. Make the eyes enormous and close to the top of the head so that your child has lots of room to look around-- dark tulle, mesh or sheers work great. Cut the eyes out and try it on so you know how to adjust them, then sew or glue the mesh onto the inside. When you're done with the costume, you can cut it up and use the batting as it was intended!
Shiny capes are also quick to make and can turn a fancy dress into a princess/fairytale/witch, pajamas into any one of a billion superheros (stencil the logo onto the shirt and you're done!), or a suit and tie into dracula/magician/some dude from the 19th century (sure to be a hit with the kids).
Bolt carries shiny fabric, quilt batting by the yard, textile paint (for stenciling), and craft felt for all your toddlers-turned-woodland creatures. If you need a little quiet time to get this all done, head down to Modern Domestic and rent some space for an hour or two. Come for Open Sew on Friday night and you'll get a whole three hours. You may even get to commiserate with some like-minded sewists. I am going to bust out the aforementioned ghostie and see who it might fit this year.