I don't know what this says about me but I decided that what I would really most want to say to the stranger would be "Surprise me." Clearly, I am not easy to please - lol.
So... what if you said... "Okay, surprise me," and he, in all seriousness, said... "BOO!!" And then he left, thinking, 'gee, what a great opportunity to have sex with a stranger, and all she wanted me to do was 'surprise' her.