A simple passthrough fidget toy/thing that should be relatively easy to print, with pretty wide tolerances. 

I print mine at a layer height of 0.2mm with a 0.4mm nozzle (on a modded Ender 3 S1 Pro). The one in the picture is an amalgam from leftover filaments I add lying around (hence all the stringing!), and the passthrough action is fine (even though the filaments were not really calibrated).