Another last minute detail for my kid's last minute costumes: shoe covers! If you've got fleece and elastic in your stash this is way better than buying fancy shoes. I looked at the tutorials here and here, and then added details for my kids shoes.
These are supposed to be cowboy boots, although I never quite got the shape right. You can't tell here, but I the tops are angled like boots with a little fleece tab sewn over the outside edge. You know what I mean? And then I top stitched a little embroidery design. They're a little funny looking though.
These were a little more straightforward. Pink shoe covers that I top stitched some fleece dinosaur toenails onto. These were easier, plus a much bigger hit. I think short shoe covers work better.