For the past few years, my default example of a weird thing you can learn to observe skill development (and thus potentially abstract general principles) has been close-up magic.
Lately, I've found myself surprised at getting quite a bit better at something that's also sort of arcane: arcade dance rhythm games like Pump It Up and Dance Dance Revolution. It helps that there's a free Pump It Up game at my university, so I can play it a lot. And I've been playing it a lot.
There was a little bit of deliberate practice involved, where I looked up the foot patterns for a few songs and played them (tapping my feet on the ground, sitting in my chair) at 0.5x speed so I could better move my feet.
But most of it has been just brute playing.
For these games, the difficulty level goes quite high. And something that has felt like an art more than a science has been knowing when I can advance to the next difficulty level. It doesn't help that different songs at the same difficulty level can still be relatively easier or harder. Sometimes, I'll try to challenge myself and play something harder, and I'll completely fail. But sometimes, I'll realize that the song is doable, and then I'll just keep playing at the new level.
Anyway, I'm not gonna analyze this too much, but it's pretty cool for me to see that I finally got good at this thing which I associated with a cool aesthetic growing up.
I think, in general, it's made me better at rhythm games of all sorts.
This is especially interesting because, growing up, I was terrible with rhythm games. I hated them. There's a terribly annoying part in Ventus's story in Kingdom Hearts Birth By Sleep where you need to shoot ice cream scoops. I got stuck at that part, and I just couldn't progress the plot anymore. Anyway, now I don't suck at rhythm games.