my first idea was to see how much you could touch a stranger without them noticing and take a picture of how 'far you got'. the levels would be touching with a single finger, pinching, cupping, hair touching, or skin (which ive never gotten away with).
My second idea was to pretend to take a picture of someone or a group smiling, but actually be filming a video. you would film the whole "can i take a picture of you? okay say cheese....got it thank you" bit. i think this idea is interesting because in a "say cheese" sort of picture you cant see how reluctant a person is before and after. I could even be filming if they turn me down.