Use spray paint to create large images or murals on a polycarbonate sheet. You can use masking tape to mask off certain areas of a polycarbonate window so that the rest of the window does not get any paint on it.
Cut out geometric shapes from construction paper or cut out heads and then draw faces on them with colored markers and tape those onto the polycarbonate sheet.
Apply an anti-graffiti coating to the polycarbonate sheet if you want to continually decorate the sheet with different patterns or images. The coating will hold your decorations, but allow you to quickly remove the paint with a simple paint remover spray or liquid.