Idea #1: A game where you create an avatar/character that you can play/interact/train and when you scan QR codes they would either fight to get stronger, get an item like for getting stronger or help them in someway, and do quests which gives out rewards.
Idea #2: Another idea for QR Code useage would be somewhat based on puzzles. The QR Code that is scanned will give out a random puzzle/riddle for you to solve. Sometimes timed sometimes not. Once completed the puzzle will be collected and be stored for a later time if a person wants to do a puzzle once again.
Idea #3: A QR Code that is a reference to a shooter. You scan the code which becomes a crosshair and you must shoot the targets that are coming after you. This is a timed thing so you have to keep shooting until the timer runs out which will grade the number of hits you had scored.