Before you get to the point where you have no more conversation, end the talk. You have to go, you have to (insert random BS here), just always better than "So...".
As for having something to talk about, pay attention to everything she says, and try and read between the lines. Yeah, tall order I know, but it has to be done if you hope to talk to them on a regular basis.
And BTW, there should be no worries on your mind if these girls are coming up and talking to you of their own volition, then they're taking the initiative.