Chalk on Sidewalk (34" x 50" [86cm x 127cm]. Apparently sidewalk chalk makers don't think kids need dark colors. This one is right in front of the entrance to my house. Luckily it's close enough to Halloween that the neighbors will think it's just a decoration and not be too freaked out. Thanks to Jessica for providing the suggestion and materials!
UPDATE: It started raining about 10 minutes ago and this has totally vanished (before the neighbors even got to see it)!