I came across an idea on Pinterest the other day; letting your kids paint on windows. I mixed washable tempura paint with clear Dawn dish soap, about a 50-50 mix. The soap thins the paint so it lasts longer and also makes it super easy to clean off.
This is a messy activity, no getting around it, but the mix of washable paint and dish soap eliminates any hard scrubbing or stains.
The end result is really beautiful; colors back-lit by sun streaming in the windows. This is Juliana's masterpiece:
I think it would be fun to make some black paint and paint intersecting lines over the whole window, then have them fill each section with a different color. Custom "stained glass!"