That's what the EEPROM is for! However you still have to explicitly program it differently for each device, which is a pain.
Or you could just add one of these I2C GUID chips to the project perhaps?
https://www.microchip.com/en-us/product/11AA02E48
Seen it used in the Nanode for a hardware unique MAC address, but you can just treat is a 48bit GUID on a chip, the manufacturer has done the work for you.