My new thing in public settings is to lock eyes with someone and just stare at them blank faced. The longer you do it the more confused they get. I imagine their thoughts, wondering if I think they're attractive, if I'm just staring into space and they just happen to be in my space view, if they have something on their face. Every now and then I'll make a face, maybe a sly smile or a slight disgust. Or both! Keep them guessing!
A good stare is enough to make anyone squirm.