This barely qualifies as a hobby, but at some point I decided to learn how to count in binary on my fingers. It’s handy if you need to hold a number in your head for a bit and can’t write things down, or to count past 10 visually on your fingers for somebody. There are probably YouTube videos on it. I literally can’t remember where I learned, but I practiced a lot when bored in church. It’s relatively non-disruptive and practicing can eat a decent amount of time.