I like the Rhetorical Jeopardy game . Give people, especially strangers, what appears to be an answer to a question they didn't ask and just watch them.
My favorites are the time, temp but the best is to just say a day of the week, that it isn't, and wait for a response or use a name like you are trying to guess their name.
But here's the best mind screw that I have found. Go up to a total stranger, this works particularly well on women, and start a conversation as if they're someone you know. Start with "Judy, how are you? (critical here not to let them get a word in yet) I haven't seen you in a year, how's Frank and the kids"? Now wait. Denial is coming unless you've accidentally hit the right name which has happened to me twice but they will still deny being the person you know. You register a look of shock "Oh, I am so sorry, you could be Judy's identical twin. I am sorry to have bothered you". You walk off and go about your business but they're thinking 'there's someone that looks just like me? And evidently close by'. I defy anyone not to let that marinate in their head,