As a person with a fondness of things in miniature, I imagine it would be easy to get swept up in all the fun tiny details of making a farm-based marble run like this one by Katie of matsutake for Whip Up. I love how she created the corn maze and all the little farm obstacles along the way. See the project over on Whip Up. It makes me want to try all kinds of themed marble runs (outer space, of course).
How-To: Cereal Box Marble Run
Corn Maze Marble Run