There are only two requirements: there needs to be a way to evaluate progress (i.e. reading books wouldn't really count because you can hardly 'improve' at it), and the recurring mechanic needs to be fun in some way.
This might sound very esoteric so here are examples of what I've done so far:
-League of legends (peaked top 50 euw s2, master/challenger every season afterwards until...s9?) but it is pretty boring now
-Bouldering (7b+ indoors after 6 months) but it is not maintainable because I used to go for 2-4 hours every other day, which is impossible while working full time.
-Poker, studied and played until I was marginally profitable online and crushing live low stakes. Again, I can't do this while working full time because it is very time consuming, and you get bored of the game pretty quickly.
The activity can be physical or mental, I don't mind either. I guess some obvious candidates would be chess and rubik's cube but I never found any of these fun enough to 'spam' until I got good at either.
Any ideas?
Anyway, I find your approach of 'hobbie' a bit strange. I feel like hobbies are something that closely relates to 'you'. I mean, if you like sport, you can go to dance, mma, shooting, trail, basket, hand, etc. Possibilities are endless. If you like intellectual activities it can goes from video games to engaging in neww studies (physics, neuroscience, etc). You can go in artistics things like painting/sculpting/photography/street art/... You can go on blacksmithing/woodturning/jewelery/etc
I think the best question to find your hobbies are: "what do i want to become ? what time do i have?". I know someone who's struggeling with an urge to find something to do after his work an who finds him unable to engage in any activity because he is not driven by wanting to do something for a purpose but just to have an activity. Having commitment for something you do just to makes your life less empty is really hard. It's easier to find hobbies if you have a goal.
I don'y know if it makes any sense to you or even if it's revelant, sorry if it's not the case.
EDIT: just posted on HN : https://idiallo.com/blog/im-bored-what-do-i-do
Good for low-level general fitness, balance, coordination, timing, and forming the ice at parties. No upper limit as to accomplishments, but plenty of low hanging fruit.
There are probably 1000 tricks/patterns you can do with just three balls, ranging from easy to professional skill required.
Practising the right thing for five minutes, three times a day, you can become a capable juggler in a couple of weeks, and a decent, impressive juggler in 6 to 12 months, depending on your general coordination. But if you've done a lot of bouldering you can probably learn to juggle three in an afternoon, and get to some interesting patterns in a couple of days.