I believe that you can take smiling at someone one step further with a wave:

When you wave at someone, you kind of force a partnership with them.  Often times people will “check you out” in a judgemental fashion…looking you up and down.  When you make eye contact with them, they may look away suddenly, disengaging.  This is a one sided arrangement and doesn’t feel good for either person.  When you wave at them in that moment, you engage them first by surrendering with that “first move” so to speak.  They are now “in the moment” with you.  In this moment we simply acknowledge each other in this realm…we’ve connected.  Both of us get something from this engagement that is positive.