I think the whole fidget spinner thing is corny, but my kids are into them right now. I had a set of bearings on my desk, and for whatever reason I found myself spinning them as I work. So I gave in and designed my own, but I just wanted a disk. Nothing fancy.
This design uses pennies for perimeter weighting, because you can't get much cheaper than 12 cents for weights vs. $1 or so for three cheap bearings. The center bearing is a typical 608.
I also didn't like most of the caps I found, so I designed some simple ones that press together and are quite thin. I made sure there was enough of a shoulder on it to keep the cap from rubbing on the bearing surfaces.
The outside of the disc spinner also features some nice knurling, so it's easy to spin fast.
The pictured print is out of Onyx, 50% fill, and 0.1mm Z resolution.