solving Rubik's cubes is how I keep my hands busy, it's extremely satisfying and tactile. I do it constantly at my desk, basically whenever I'm thinking or watching something.
I also love putting a blanket between my legs and rubbing them together, it feels like it releases all of my stress and anxiety and gets me super cozy.
I can see how you would think that, but it's really both. The mechanical sensation of turning the cube is very satisfying and I make a lot of quick flicking motions with my fingers in predefined patterns. I solve pretty much passively, automatically by now, and never time myself.
I think nearly anything can be a stim. It's really about what the experience is like to the person doing it, not whether it looks like stimming to you from the outside. Please try not to invalidate anybody's stim. If you're confused, ask questions about their experience.