Does anyone have any tips or lines I can use on girls (or people in general) but more so girls that is not just saying "hey" when passing by. So I see girls I know from class around campus often. One of my major problems is that I tend to be shy and just quietly say "hey" and muster a half smile. Sometimes I just mouth it. Usually it is because I am stumbling because I am seeing them unexpectedly and I have hard time being spontaneous on the go

What are some go to ways of greeting that come off friendly, happy and confident that would work on girls when passing by. Something that could lead into a stop by short conversation but always leaving a good impression for next time bumping into them.