Make a water wheel - perhaps it would work with marbles in a marble run too
222年間にこれほど退化した「特許図面」の歴史：ギャラリー « WIRED.jp
How To Stencil With Bleach
From maps to mazes (homemade maze out of straws and box) by Teach Preschool
Learn how to make a kaleidoscope in this fun STEM/science activity for kids. It's such a fun way to explore light, reflections, and symmetry! ~ BuggyandBuddy.com
How to make a water wheel
The RunnerDuck Waterwheel plan, is step by step instructions on how to build a decorative waterwheel.
Changeable Magnetic Marble Run, be sure to test the weight of the component to the strength of the magnet so it will hold.
Another pinner says: How Many Drops of Water Can a Penny Hold? My daughter did this as her science fair project last year and it was very popular. She even used water variations (Kool-aid, salt water, etc).