A very simple activity, you just need various colors of Post-Its and window or wall space. We did this program a year ago, and we found out the Post-Its don't adhere to our painted walls very well. Large glass surfaces work best. Otherwise, a large roll of butcher paper comes in handy.
It's helpful to have a few examples or guides, or even provide graph paper so the teens can plan out their art before putting it up. We'll be leaving these up through Teen Tech Week - they're perfect decorations!