Best two odd things I've seen go through the mail:

A NYC subway token (these have holes in the middle) with
a tag tied to it, the tag had address and postage. Sent by
a friend of mine to a relative of mine.

A four inch pumpkin. I glued a priority mail stamp to it,
addressed it to my cousin's husband with a sharpie marker on
the pumpkin itself and sent it for his first Halloween in
the US. Received without damage.