Emart, a large Korean discount chain, launched a lunchtime promotion where shoppers could scan a shadow QR code, using the sunlight and shadow only available between 12 PM and 1 PM. The shadow code gave shoppers access to coupons valid only for the hour.
Skip straight to 0:30 to see how the shadow QR code works. Brilliant combination of new technology & nature and limited-time-offers. Of course, you need to have a few hours of guaranteed 'some sunshine' during the day. :)