Styrofoam, like the stuff from take away containers or from the grocery store, can be used to make your own stamps. (This site is in French. I tracked the photo to http://www.funnycrafts.us/printmaking-ideas-for-kids/, but there are no further instructions there. I guess they figured the pictures said it all.)
Popped Bubble art: Simply color bubble mix with food coloring and blow bubbles onto paper - as the bubbles land and pop, they create beautiful works of art. It goes without saying that this project is best done outdoors. A great one for those lazy summer afternoons.