Let's make some stuff which is both compatible to Duplo & Hubelino: marble run
Buy at Hubelino or print a duplo compatible marble track set: like joachim67, thecrash74, a marble collector from wuz23 and some extensions from soundTricker.

2 Dual color marbles:

So let's first print some 2 color marbles (with text) and measure them:

• Hubelino marbles: 24.2 to 24.5 mm (measured 4 balls)
• Printed 2 color marbles: 24.1 to 24.8 (measured 6 balls)
  Okay, need to sand them a little bit better and should have calibrate both nozzles before printing, anyway, the printed balls do roll nicely over the tracks.

Simple wedged tracks:

Ofc we need more tracks. Going for sense & simplicity, I like these tracks from wuz23 a lot. So decided to do some laser cutting on this one, although these could also very simply be printed. Could be used w/o elevation height, since there is a wedge shape inside, which makes the balls roll anyway. Added 2 versions both dxf & stl. Making these from maximum 4 mm plates.

Flex track v7 (updated 20181201):

Add some more 3D tracks for your marble run: the marble flex track.
Printed some flex tracks, which works fine.
Using: PLA; 0.8 nozzle; 0.2mm layer height.
Version 7 is flexible from design and does not need special materials.
Version 5 is also still available, see the nice TPU make from Gogo83 and hd42.