You could put your onion address into an “oh by code”[1] and just write it down … or chalk it on the sidewalk for someone to see … post it on a physical bulletin board.. hold it up on a sign…

This way you could establish communication with an unknown future party, totally offline.

[1] https://0x.co

Thanks! My realistic use case is that I am already speaking to someone who I know and trust, so ideally exchange credentials in person. A preferred out of band secure messanger of choice is probably fine.