I saw this article, Those Darn Spinners are Going to be the End of Me, the other day. This got me to thinking and wanting to play.
Here are a couple of fidget spinners that I designed to make experimentation easy. I'm using fender washers for weights. Each spinner can accommodate a variable number of washers. The one that looks like a Tie Fighter also allows you to position the washers varying distances from the center (you'd need an extra set of nuts though).
I used standard skateboard bearings. The grease causes these to slow down too quickly so I dropped them in some Acetone first (a quick google search will show you many ways to degrease bearings). I also found they spin longer of I popped the metal covers off. All the YouTube videos I saw for this showed using a knife to do this. Beware you may cut yourself... I'm not saying I did, but...
There are a number of bearing caps you can use. I used ones from this spinner. The nuts and bolts came from Nut Job by Mike Mattala.
I've broken several of these after they've been dropped. I thought the arm would be the weakest part, but it turns out to be weakest where the arms meet the ring. I just uploaded designs with thicker rings. I can now also confirm that they don't break at this point any more.