Look! I'm so nice, so generous and giving, that I'm even providing you with a pretty shoddy picture of the artists in action. Bonus: broken screen door in background.
Gather your supplies: paint*, marbles (buy a cheap bag at the craft store), a pad of oversized watercolor paper (use that 40% off coupon; those babies are expensive!), and some sort of tray. If you use, say, 11x14 paper, a cookie sheet with edges might do the trick. We used bigger paper so I directed the Professor to create some huge cardboard trays out of a big box we had lying around. "Your wish is my command, O Queen," he said cheerfully. Or something like that.
Dip your marbles in paint. Plop them on the paper one by one and start rolling and tilting your tray. Or, you can use several marbles all at once and really stir up some craziness!
Ta-Da! Wall Art of which Jackson Pollock would be proud.
By the way, in case you haven't already discovered it, The Crafty Crow is a fantastic place to grab last-minute craft ideas. I use it regularly for my own kids and my Sunday School class; in fact, the marble painting idea was doubtless nabbed there. Hooray for artsy people sharing their nifty ideas!
(* edited: TEMPERA paint)