I guess that those days, it is possible in most countries to print your own, valid rubber stamps.
Therefore, I have printed out a couple of dozen stamps with Bitcoin-pictures. Which was fun.
But this wasn't enough - call me "Daredeviil"
I printed then a couple of rubber stamps with a private key on it and sent them to my daughter. She was delighted to see money delivered this way.
A less risky version was to send a QR-code splitted on two stamps and send two postcards.
Just a little bit of fun, while knowing that our postal service still doesn't accept Bitcoin :-)