Here's a fun Halloween mail art project for you from Patrick of Edelweiss Post. He's got a clear postcard idea that he sends to friends as the Invisible Man. I think they'd also work as a Ghost Post or other unseen ghoulie mail. You just need clear plastic sheets, scissors, a permanent marker and stamps. Cut down your plastic sheets (I use transparency overheads from the office supply store) to postcard size. You could measure it out, but I just hold up a regular postcard and cut around it. Then write out your message in permanent marker. Remember to let the marker dry so that you don't accidentally smear it. I hate when that happens, don't you? Put on a stamp and you're ready to send your CLEAR message. I'd recommend using letter postage instead of postcard postage due to the special handling the people of the Post Office may need to do to route the transparent cards correctly.
L.W.A. member and friend Kimberly also has a take on this idea using packing tape and glitter with sequins. The only good glitter is contained glitter. (Seriously, that stuff is a hazard.) The look of stuff suspended in clear matrix is really neat. Good use of the Batman stamps too!
P.S. Check out Patrick's Movie Monster stamps. So perfect for the spooky season!