When I was a kid we had an old Chevy we dubbed "The Old Red Bomb." What finally sent it the scrap yard was when my Mom watched my little brother and I disappearing below the seat back (before seat belts) and jumping up to stare out the back window, giggling hysterically. It took her a couple of trips to discover the nature of our fascinating game. We'd discovered a hole in the floorboard, just under the edge of the front seat. We dropped whatever we could fit through the hole, then pop up to watch it bounce behind the car.
Y'know, pencils bounce better when you drop 'em eraser-end first ... and marbles are no fun at all because they roll away faster than you can track 'em!