Low friction design with sufficient play to run very easily, just like a fidget spinner.
It doesn´t require any additional parts such as bearings and prints as one part.
I play with this a lot while working on my PC, it´s kind of relaxing.

Gear data:
Module: 1.7mm, Pressure angle: 40 degrees, Helix angle: 40 degrees, Height: 16mm
Teeth: 33, 12, 9
(I created this using a home-made parametric solution in Fusion 360, not a plug-in)

The sun gear tightly fits a 6mm hex key.
If you scale it down (50% shown on photo) you might want to reduce the horizontal expansion in your slicer tool to adjust the gaps and ensure smooth running.

UPDATE (February 2, 2021):
Finger rest added. Print 2 and plug them in for more comfortable spinning.

UPDATE (June 28, 2023):
New version with improved gears and tolerances

HAVE FUN and consider sharing your make with the community!

Anycubic

All-metal Mega

No

No

0.2