I try to avoid anything too serious that may seem like he/she is in a job interview... keep things light-hearted at first. What CD is in your CD player right now? What kind of shenanigans do you get into on a normal weekend? Things like that.
Don't "act" interested if you're not. Just ask about things they like to do, where they are from and what kind of work they do. Don't get too nosy but you should ask some questions to get to know them.