I don't know why younger people like these objects so much, they remind me of cat toys :D
Anyway, I like the challenge of making something that is not my cup of tea, and this is the result. If it's a good fidget thing or not, you decide. It turns all around one direction or another, it makes clicky noises, it's decorated and stays in the palm of a hand.

The assembled object measures ⌀50,8mm x 18mm. It requires one M3 20mm screw and one M3 bolt.

I designed multiple gears so that you can put inside the one you prefer: clockwise or counterclockwise, in three main different shapes and resistance. I also added a customizable gear that you can modify with your favourite design software.

To assemble this Ratchet Fidget you need to print the Base and the Wheel. You also need one gear: you can print them all or select one among the list.

The clockwise type of gears will make the Wheel turn only in the clockwise direction.

The counterclockwise type of gears will make the Wheel turn only in the opposite direction.

For rounded objects like this I prefer to use a circular top and bottom fill pattern in Prusa Slicer (= external fill pattern → circular; in Simplify 3D), for aesthetic improvement.

Here is a short video of the Ratchet Fidget: