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).
