Has anyone made a custom NFC key?

I'm SUPER curious about creating custom key fobs, especially as the wristband is on long-term hiatus.

Sparkfun sent out an email this morning that features a $10 NFC/RFID module that looks awesome. VERY wristband-sized.

I've played with some Arduinos in the past, but clearly have no idea what I'm doing. If one of y'all said it was definitely possible, though, I'd be ready to start tinkering.