Fun uses for glass marbles?

My girlfriend and I were gonna spend a saturday making things with marbles. Sounds awesome right? haha. We were thinking we might make some cool looking stuff by hot gluing them together or heating them up and putting them in cold water (I hear this makes em look cool). I think the only way to melt marbles is with a kiln but if there were any other ways to melt them, that would be good to know. Anybody have any fun and practical uses for those old marbles that just sit around gathering dust?