I got a a lot of 12mm ball bearings really cheap off ebay a couple of months ago, for a project where I really only needed a few really. So what do you do with 90+ steel balls? Lots of spinning tops.
This is version 2.0, which works really well and prints pretty fast. Ball insertion, no pun inteded here, can be a little tricky. If you warm up the print, it's a little easier to get them positioned correctly.