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I have a couple of sets from Fischer Technik and can't recommend them enough if watching marbles roll in an infinite cycle is your thing :)

Really well-made, excellent instructions and a very Lego-like experience.

If you've got a 3D printer and some Lego lying around, you can print Lego-compatible marble run parts, one example: https://hackaday.io/project/165034-marble-run-building-brick...

Fischertechnik is a great engineering gateway drug for kids