Use water paint to create artwork on the windows that washes off whenever you want to clean up or paint something new!
For Halloween, I made a series of pumpkin portraits on my living room windows.
Water paint needs a couple layers for the colors to appear bright. Allow each layer to dry completely before working on it again. Work lightly, carefully, and quickly to avoid removing previous layers. I layered brush marks of varying colors to create the coloured effect I wanted
Use dry erase markers if you need to make some pre-planning marks.