I had a lot of fun with mail art in the 1980s, sending collages and small sculptures to friends. In 2016 I got the bug again and sent hundreds of diorama boxes to friends via US Mail.
Each box had a clear plexi cover (or in one case saran wrapped) so the pieces were visible at all times – a joyful WTF moment for you and your mail carrier.
On the back / address side were carefully selected stamps and each return address said “of Course I’d Like an Explanation of This” and “retaliation appreciated.”
I received some great mail art in response.