[tomsaffell]

Good question, but I think the answer is in the question:

  I want to __ask__ people what they __care__ about... 
  How do you open __small talk__ ...

I would make small talk about something small, like the weather (I'm British :), or about whatever common experience around us we're sharing (the music, how bumpy the bus is, how full the flight is). Taking a random walk through conversation-space should fairly quickly get to something that they care about, so long as they want to talk, and have their self-interest-heuristic turned on.

So I don't think you need to seek what they care about. You just need to make the conversation worth continuing and it will get their on its own.