You don't really need an opener, other than the casual "Hey, whats up?"
I tend to say an opener based on a girl's facial expression at the time and I make assumptions based on how she looks. For example, if you see a girl and she looks extremely happy, I would say: "Hey, you look extremely happy today, what's going on?!" OR "Hey, you look a little down, what's up?"
It doesn't really matter what you say, just ensure that your opener does not allow her to say simple "Yes" or "No" answers, and you are in!