This activity wasn't planned at all. Later this week you will see another activity that uses these same page marker post it notes, and this activity just sorta happened after we finished that one.
You will need some page marker post it notes and a window.
You can create something literal like this flower.
Or keep it abstract and play with all the colors.
These didn't leave any residue on my windows but if you left them up for days and days you may have to give the window a wipe.