Happy summer to everyone! Now that the kids are out of school it is time to find them things to do. Letting them draw on your windows sounds like it's breaking the rules and thus an exciting way to spend an afternoon. If they're old enough, challenge them to write backwards so that it's legible to a passer-by. Bright Stick wet-erase markers work well, so do Chalk Ink markers. Both wipe off with a damp cloth.
Window drawing is a fabulous way to show off your child's creative spirit, give them something to do on a rainy day, and make everyone who walks by smile.