Making people second guess themselves, especially before flipping their tables, because Too many people think they're right all the time. Or make compulsive liars unable to lie, without a good reason of course. Or perhaps giving people inhuman levels of focus, or play around with thinking styles, perhaps thinking differently could boost one's logistical reasoning. I could also probably cure sociopaths, schizophrenics, or other mentally ill people.
Or I could make people obsessed with grilled cheese à la the sims, or make them think that one object is another, like making them think bananas are guns, credit cards, keys or something. Imagine a guy waltzing into a shooting range, and just standing there shaking a banana, then getting upset because he "isn't hitting the target", then he tries to pay for another gun, but he just pulls out another banana and goes "just put it on my card.", then after getting kicked out he tries to unlock his car with a third banana to no avail.