Customizable Excenter Ball

This is a toy ball with a decentral center of gravity that makes it behave in an uncommon way.
When the center part is printed halfway put in something heavy on one side (e.g. a nut or some piece of scrap iron). WARNING: Check if the overstanding piece will not crash into yor fanduct before you put it in. When finished glue the caps on (~33% infill) to make a complete sphere. You might want to lock the enclosed mass so that is does not rattle and thus does not quickly stops movement (e.g. with a folded piece of paper).
There's also a fill option but then you need to print the center part near 100% infill and will get at best 1/5th the effect of an iron fill.
density iron: 7.874 g/ccm
density plastic: ~1.2 g/ccm
Give it your cat to play or find some other unique application.