As a mechanical engineering student, I thought it'd be interesting to try to make a fidget spinner which uses planetary style gears, so here you go! It can be spun conventionally like a fidget spinner, or you can hold the rest still and spin the gears! Or roll it on your back for a back scratcher, etc. See print instructions below. The design uses tight tolerances and the actual gear involute profile, so it needs to be printed correctly to work.

1. Make sure you print each gear by itself if your printer has any remote stringing issues, and with minimize perimeter crosses setting checked. It's difficult to sand in between gear teeth, so if any strings get there the gears won't mesh.
2. You'll likely have to drill out the holes in the center of the planetary fork and planetary receiver (use a 1/4 inch drill bit). The bottom layer is often wider than the rest and will interfere with the hole.
3. Make sure the bottom side of the center gear is on the opposite side of the spinner as the bottom of the other 3 gears. Tolerances are tight, so if the wider bottoms attempt to mesh it can jam the gears.
4. You'll likely have to sand the planetary fork and the planetary pins due to stringing. The gears should all spin fairly well around these. Print two planetary pins at once so switching between the pins will prevent overheating issues.
5. The exact tolerances required likely change with extrusion settings. If something doesn't fit, something doesn't stay together, etc try changing the scale by .01, etc. The tolerances come down to hundredths or thousandths of an inch.
6. Do NOT print the full planetary_fidget_spinner stl. I put it there for visualization.

Lastly, I'm trying to fit a pin in the middle so that the sun gear/middle gear will remain stationary and the rest spin relative to the pin, and I will upload that as an additional pin in the near future.

Update 1: The cap on my spinner started falling off after some use, so I'm uploading a slightly smaller version of the cap. You may or may not find it better depending on your extrusion settings.