Instructable user seamster has created a pretty neat marble run from a single 8 foot 2×4.
The whole thing is 22 steps long, but it’s very well documented, and worth it. I think it’s superb, the 2×4 is a staple of modern wooden construction. The cost almost nothing, and with really only a bandsaw and a table saw you can make this entire project. It’s a perfect thing to get kids interested in engineering. Especially the part where adding all the cross braces greatly increased the rigidity of the contraption. Plus, marble runs are just plain fun.