Has anyone done any experimenting with short-range offline communications without radios?
Things like the Pirate Box:
https://piratebox.cc/faq
https://null-byte.wonderhowto.com/how-to/build-off-grid-wi-fi-voice-communication-system-with-android-raspberry-pi-0296270/
The ESP32 board:
https://null-byte.wonderhowto.com/how-to/enable-offline-chat-communications-over-wi-fi-with-esp32-0296277/