Door hangers have a variety of uses.  Missed delivery, do not disturb, parking permits… whenever you need to leave a message on something like a door or the handle or a machine, door hangers will do the trick.  Made from a thicker stock of paper (or even polystyrene sheet), the door hanger has an opening and a small access channel cut into the material.  This allows you to put it onto a bar or doorknob with slight pressure, and will keep it in place.  Door hangers can come in a variety of colors, be printed with a lot of options and are inexpensive enough to deploy on a large scale.