With a little ingenuity and some Rube Goldberg styling, your children can tape together a runway that's made for marbles.
Keep the following basic guidelines in mind: If you slip one tube inside another, be sure the larger tube is on the downward side of the run. Make sure the end of the run is at floor level. Flying marbles can be dangerous. A combination of closed tunnels and open sections where kids can see the marbles moving makes everything look a lot more interesting. Larger marbles tend to roll better than smaller ones (rubber balls will work well, too).
Help the kids set up a landing for the start of their run. A stool or the back of a chair or sofa works well. Then, arrange several stacks of books, cereal boxes, toys, blocks, wastebaskets and other objects for support, so the marble run will slope down toward the floor.
Start joining the tubes into a maze, securing the connections with tape. Wrapping-paper tubes are the most efficient structures for your marble runway; paper-towel tubes are good for the shorter runs, and toilet-paper tubes make great corners for turning.