I have been doing art mail for years. My favorite way is to make postcards from the materials that are in front of me. I mail them to myself several times so that I get multiple stamp and canceling images. Yes, these postcards are completely legal. If a postal person tells you that you cannot do this, just be polite because I promise you, you can. Once in Tokyo, I was having trouble getting the hotel personell to mail something for me so I just went outside and pushed it into a mailbox. It was a red laquered paper fan that I had addressed to myself. It came a day or two after I had gotten back to the USA.
These postcards have been sent to the Art House Coop in Brooklyn as part of a mail art project that they will make a book and exhibit from. If you want to "play" my address is on the card below .