What if you and your young kid are alone at home, and you have to cook the dinner, but the kid likes to mess up the whole room while you are in the kitchen?
In that case you bring your kid with you to the kitchen, but making a playground in the kitchen is not the best idea. In order to invent something for such a case, I came up with this marble track that fits on a refrigerator door.
It's not new; I know. There are plenty of similar ideas, but most of them aren't 3D printed. And that's exactly the USP of this track; as long as you have a 3D printer with some filament, you can easily expand the track and customize the set. And if you have some drawing skills, it is even possible to design you own module to extend the variety of the total track.
I already made a few modules, like a seesaw, slingshot, looping and a basket.
The holder contain a 20x2 magnet; these can be (online) bought at lots of hardware stores. Furthermore, you need some marbles with a diameter of 10 - 12.5 mm or in imperial units: 2/5" - 1/2" (Note: don't take smaller of larger marbles; they can't go through holes or stay on tracks)
Designed with SolidWorks 2019 SE. Unfortunately, I don't have a working printer at the moment, so, couldn't test if everything works as expected.

Print instructions

The tracks themselves could be printed with a flexible material, such as TPU in order to bend them.
I designed them to print it without support.
Magnets can be press-fitted or mounted with glue.