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.)
This DIY art project idea is really easy, so much fun, and makes beautifully colored notecards. You just need a few simple supplies you may already have!