This is my take on the classic Spirit Slates routine. A Priority Mail envelope (or any rigid, large mailing envelope) is torn open and a stapler and magic marker are dumped out. The edges and flap of the envelope are torn off so only the front and back sides remain. These are displayed and numbered on all four sides. They are stapled together around all four edges, with the marker trapped inside. They are ripped apart and the marker falls to the floor, revealing a big drawing or message on the inside!
This is great for parlor or stage and can be used to reveal a mentally selected word, as a drawing duplication or just to make a fun message appear.