Using certificates for SSH authentication

I don't think hardware cost is much of an issue, even at single unit prices they're less than a tall flat white and presumably somewhat less in bulk (https://www.google.com/search?q=nfc+reader&tbm=shop). Most mobile phones have NFC already, so adding it to a laptop or an access terminal isn't much of an ask. There are keyboards with integrated ID card readers which are basically the same thing.

On the other hand, Bluetooth, WiFi, USB (for key dongles) and QR codes are all somewhat more commonly available and could do the job (perhaps combined with "enter this code to confirm"). Android phones use QR codes to share WiFi connection details, for example, and WhatsApp uses them to connect to the web client, which is a pretty similar authentication pattern...