...sidewalk chalk paint!
Homemade sidewalk chalk paint is a VERY easy recipe, but it's more of a niche art supply here, in that the girls tend to forget all about it for months at a time, and then, goofing around outside while their dad cleans up the front porch (it had reached the point where, even though I'm not all feng shui and I don't always tend to the appearance of our home from the outside, I was starting to worry that trick-or-treaters would be too frightened to approach our house!), one of them discovers the sidewalk stencils, and then it's as if a lightbulb goes off--
"Sidewalk chalk paint! Yeah, let's do it!"
And there goes the afternoon!