I got bored with fidget spinners pretty quickly, until I found this one: thingiverse.com/thing:1897308

Specifically the 3-hole version.
Put a bearing in the middle hole and in the hole in line with that, but leave the off-center hole empty. Now you can use this fidget spinner completely one-handed. You can either put a finger in the empty hole and spin it like you would a ring of keys, or, because it's weighted off-center, hold the middle bearing and spin to your hearts content.

You can also get some really satisfying wobbly-spins when flicking it two-handed, but the one-handed use is what really seals it for me.