[elric]

+1 for martial arts in general. Depending on the art it might not be niche (I'm sure karate and taekwondo are too big for that). But things like HEMA, iaido, eskrima, all kinds of archery, ... are great fun and typically come with smallish & fun communities.

Edit: before you think these arts are immune to tech, I once had a student who built a (truly awful) sword fighting "robot" to help train deflecting strikes. Not quite up to par with Dune's robot swordmasters.

[tmtvl]

Seconding or thirding martial arts (MA). I do Kung Fu, but I'd bet all the internet cookies there's at least one Brazilian Jujutsu practitioner on HN.

On the point of tech in MA, I recall seeing a documentary where some people got a Xing Yi Quan master to perform a form in mocap gear so they could analyse his movements. I think there's still a lot that kinesiology could learn from the way different people move (especially from Inclusive Karate practitioners).