We made a bunch of these... 
https://i.nostr.build/LLw6.jpg

All we did though was throw a NFC tag inside each of them and make them point to the nostr.world website. We made individual customized ones for every public speaker and they pointed to the user's npub. 

Would love to see a NFC enabled way of signing events,  nostr:npub1c878wu04lfqcl5avfy3p5x83ndpvedaxv0dg7pxthakq3jqdyzcs2n8avm brought it up and I think the potential for a NFC card that's a Boltcard, has your npub and lets you sign events would be really really cool. Probably too early for it's time though.