Vampire Vision from Center for Game Science is a fun and challenging spot the difference game. You are a vampire hunter. Your job is to use the clues given to you during a tarot reading to rid a village location of vampires. The cards might reveal that vampires will have red eyes or run in the dark. You then take a field-wide view to point out who the vampires are, stake them through the heart, and save the village. The goal of the game is to be a sort of vision training exercise. You working on the ability to follow multiple objects at the same time and make quick distinctions between them. Hidden behind the novel vampire hunting/shifting attributes theme does wonders to make it fun.
There is no experience curve as you could randomly be placed in the hardest style of recognition for you after the initial training stage. Maybe you do great identifying which vampires are dressed different, but struggle to tell which vampires can move quickly at night. The game makes it challenging enough to feel like you should invest your time, but simple enough to progress that anyone could take a stab at the game.
For a fun time-waster with some good novel concepts, you should try playing Vampire Vision.