Device-to-device audio communication (think QR codes, but made out of sound) seems very underrated. Much better UX than QR codes or NFC because you don't have to worry about putting your phone in the exactly right place or pointing it in the right direction.

Anyone here with more detailed takes on this?

Surprised nobody mentioned Clinkle yet. https://d3.harvard.edu/platform-rctom/submission/clinkle-how-to-go-from-a-soon-to-be-unicorn-to-complete-failure-in-one-year/

Software Engineer @ Cadence Design Systems. B4: Stanford EE / Illinois ECE