I am a grammar nerd of the worst sort. It's not that exciting, as hobbies go, and I can turn off my error alarm on a whim, but it's pervasive if I am not intentionally blocking it.
I used to follow all the most prominent journalism on journalism as a field, but I have recently decided I don't need such a depressing hobby. I know a lot about vegetarian cuisine, and I find it a lot less discouraging to read food articles and cook. I guess it's not a "geeky" hobby, but I do my best.
Fiance is an encyclopedia of Zen knowledge. It is incredible how he can find a relevant teaching, word it for his audience's experience and education level, and answer myriad questions in an enriching way. He is also a bicycle geek if he's in the mood, and an expert on a certain type of music.