I saw a cute window cling idea at Mess for Less. You take some foam sheets (mine are from the dollar store) and cut them into shapes. Add some water and they'll stick to any window. I knew the perfect story to pair it with. Go Away, Big Green Monster! is a book where shapes are gradually added to make a monster and then gradually taken away to make the monster disappear. Our favorite part is when we get to yell "Go Away, Big Green Monster!"
After we read the story (at least ten times in a row!), we just dumped some water right in with the shapes and they were set to go. This would be great for bathtub fun too.