learning a language would be nice, but i think id require someone else who knew the language to be effective at learning it and then having some use for it. It'd be nice to be able to read some theory in its original german but I don't know if the investment is worth it.
Similarly with electronic repair, how do I ever gain those skills without having something to practice on. Which starts to get expensive. I think electronics are cheap enough today that its more expensive to buy those repair tools rather than just throwing it out and buying another microwave.
If I had money for a hobby, I would for sure be getting into stuff like drones though. Not trying to be a negative nancy, but I think we're talking two separate things. The attractiveness of brain training is that it's easy and free, at least the one i posted. You can come home, crash on the couch, and do it from your phone, 20 minutes a day and supposedly see results.