Here's a spring decorating idea...
1. Cut a petal, circle and leaf shape out of a few sponges (depending on how big you want your flower garden to be).
2. Using washable paint (watered down), have your child stamp a few circles on your window or mirror.
3. Help them stamp the petal shapes around to make a flower.
4. To finish it off, make a stem for each flower and stamp a leaf or two (ideally attached to the stem). At least one of our flowers is growing sideways out of the window frame rather than up from the "ground".
A "be prepared" note: you'll notice that at least one of my sweet children (any guesses as to which one?) decided to make a bonanza bumper crop garden (after I had told him to just stamp 2 or 3 circles...) and the huge petals I had made out of sponge didn't work. So we had to paint the petals on his flowers.
This is all very easy to get off window surfaces - just use whatever you normally use to wash windows. Or let your kids scratch all the paint off (which has been happening slowly in my house for the past several days...)