An easy-to-print fidget toy made of permanently nested rings that can tumble and shift around. Customizable (though it looks like the Customizer on Thingiverse is having some trouble with it right now, so you'll have to render the files yourself).
Two basic styles are available; the biconical style allows the rings to shift along their axis while still nesting solidly. The spherical style has a close fit.
Both styles allow the rings to tumble and be spun like in a gimbal.
I highly recommend printing the rings with colored edge bands by changing the filament during printing. If you want a transparent band like one of the examples has, be sure to set the options to thin out that band.
Also on GitHub: https://github.com/kpreid/3dp-nested-rings
This is generally very easy to print because it is entirely made of circular perimeters. The spherical option is harder because of the default thin gap between adjacent surfaces.
Try to make sure there is not too much “elephant foot” excessive squish on the first layer, as the resulting ridges will noticeably change the feel of it.