So the motto for my life should be "better late than never."
One of the theme/suggestion/challenges for October was to use a craft book that you've had sitting around but never used, more or less. For costumes, I pulled out John Marshall's "Make Your Own Japanese Clothing." I helped my 15 yo son make a kimono completely from stash. He did pretty good, and is excited to use it as a robe throughout the year. (Unfortunately, the obi was an afterthought, and a complete fail, but also from stash.)
My daughter's class requests that any costume be related to ancient Egypt, for educational purposes. She's supposed to be Anqet, the goddess of the Nile. Mostly it just looks egyptian. I used McCall's 4954 (which I've had for a couple years) and a bedsheet for the dress. The drapey/capey thing was also stash. And my husband cut an ankh from one of those anti-static mats that you put under computer desks.
My youngest wanted to be a paladin. The chainmaille hood and shirt are from Ravelry patterns I've had bookmarked for a while, but the yarn wasn't from stash. But the tabard was stash. And the shield was from that same anti-static mat and other bits in my husband's stash.
So in total I used 5-6 yards of stash fabric.