Split boarding! It's a snowboard that splits in half and lets you ski uphill, then snaps back together to ride down. Takes a lot of physical fitness and training for avalanche safety/rescue, but being able to make turns down an untouched wild mountain is an experience like no other. Maybe not super niche but outside of a mountain town probably moreso.

Also, film photography. So special taking pictures of my kid with a camera that's been in three generations of the family. The camera, lenses, everything just so much more tangible and enjoyable. Then developing it on the bathroom with some science magic and holding a physical image at the end gives a sense of a accomplishment that can't be found with a smartphone! Also not super niche, but maybe 1% of people use a proper digital camera instead of just their phone, and maybe less than 1% of those play with film as well.