Table tennis! The sport has so much depth, that you can keep learning/improving for a life-time. Whenever you think you now everything about it, there is something new to learn (spin, tactics, focus, psychology, bluffing, etc.). It's like a combination of fast-paced chess, poker and reaction testing.
That, also combined with other hobbies such as VR (Eleven VR), 3d printing (printing/designing table-tennis adapters for the VR controllers).
Love table tennis! I used to play and compete as a kid and picked it up again 15 years later, that was 6 years ago. I've played 3 tournaments since then and went from 1200 to 1600 and 1800 usatt rating. I'm hoping to reach 2000 one day. There's something about it that's highly addictive, you can't think of anything else while playing, that somehow gives me peace of mind and a way to shut down my mind, in a healthy way.
That, also combined with other hobbies such as VR (Eleven VR), 3d printing (printing/designing table-tennis adapters for the VR controllers).