Some hobbies are really excellent for generalists, because you can apply the one learning-training method to any number of bizarre or intentional circumstances that life hands you. I’m currently working on ‘select heavier than average apples with a single toss and catch each at the grocery store’ and, no joke, some guy at the store complimented this. I’m sure they were hitting on me but they noticed. Made my day. It’s the hobby that keeps on giving. (I have so far only dropped one apple and, yes, I bought the poor bruised thing.)
Other fun hobbies in the category: Diagnostic guessing, Balancing stuff, Vehicle operation, Packing efficiently (rather than most compactly), Knots (which are critical to textile and bodily repair both), Folding (or as Calvin might call it, dimensional transmogrification), Echolocation mapping (you can practice while sitting in a cafe).
At the core of this is learning how to learn, and then dedicating yourself to doing that somehow, no matter how pointless whatever hooks your attention might seem to others. I figured out at one point how to alter my visual perception frame rate to slow down and stall, just for a moment, a spinning (on high) ceiling fan’s blades. I can never get them to stand perfectly still but insomnia is cruel and the nights are long, and it’s fun to imagine what my brain is doing to brainwave sync rates across my visual cortex to make this work. (If seizure-prone, maybe don’t try this alone.) Unexpectedly, even this has had a practical value: when a passenger in trains or cars, I can consciously relax my eye muscles now and let the landscape motion blur by rather than saccade-focusing constantly. Hooray!
Every useless hobby skill has an unforeseen opportunity to be valuable :)