The simplest device that might reasonably be called an orrery. Made it a few years ago for my daughter.

For anyone else who might be wondering:

An orrery is a mechanical model of the Solar System that illustrates or predicts the relative positions and motions of the planets and moons, usually according to the heliocentric model.

Making an orrery like this would probably open a rabbit hole with my kids, but that might not be a bad thing.

How do you know about the secret planet at L3?