Here's the scenario: You have a bus driver with a smartphone connected to the internet. Then you have the passengers(who have booked seats on the bus) with smartphones as well but NO internet connection. The bus will pick up passengers at every stop and each of these passengers needs to quickly perform a "check-in" once they on board (there's no internet on the bus either). Because of budget constraints, it is not possible to provide internet on the bus or install any 3rd party device (like QR code reader) that could handle check-ins.
Ideally what should happen is: The passengers' offline phone somehow communicates quickly to the drivers and shares its boarding id (Bluetooth conns are a hassle and won't work well when multiple people onboarding). The driver phone (connected to the internet) verifies booking from the server and communicates back to the offline phone with a success message. All without any intermediary device and while handling multiple passengers at the same time.
One idea is to use something like chirp.io to transfer data using sound. Any other ideas?