Mine is mapping. I am a big OpenStreetMap contributor and I have mapped many towns near me that were previously completely unmapped.
For those who jump around too much like I do, remember:
That has some truth for career/professional skills, but I don’t think there’s anything wrong with having a lot of hobbies. Most people won’t achieve “true greatness” (whatever that means) in their hobbies whether they have one or hundreds, so why not just focus on doing what you enjoy?