When was the last time you wrote a letter — not an email, but an actual, physical, pen-on-paper (I would also accept “typewritten”) letter. And then put in an envelope. And then licked a stamp and mailed it.
Ha! Got you! You don’t even have to lick stamps any more, you just peel them like stickers!
Anyway, aside from bills, paystubs and junk mail, I can’t recall the last letter I even received, let alone sent. Oh! Christmas cards!
But if I ever do send a letter just for fun again, I think I’ve found my envelopes:

Go to MapEnvelope.com, and enter your address, along with a brief message, if you like. Because this will be on the inside of the envelope, it’s probably best to use your return address. The website itself helpfully suggests a different tourist attraction (Honolulu, the Statue of Liberty) every time you refresh the page, and if you happen to be vacationing with a colour printer, that’d be swell.
Because, the next step is to print off the map, which comes as a template that you cut out and fold up. Tape it together, and you’ve got yourself an envelope!
(I suppose if you did it inside-out, you could have the address on the outside, but it’d be a very busy-looking envelope.)
Because there’s no glue to lick, this envelope will have to be taped closed, too, which makes it more likely that people will open it using the flap, and therefore will actually see the address.
Cool idea!