This is something I came up with because I thought it was hilarious. Works in any setting, but best when you're walking down the street. Turn to the girl (say, Sarah), and with a very serious and deep look like you are about to compliment her, say "You know, Sarah...I just noticed something about you." Hold eye contact and wait. Then quickly ruffle her hair while saying "Your hair's all messed up you should probably fix it." and keep walking.