Peeping Tom counts?

Outdoor warning sirens. There are so many types out there with their own unique appearances, sounds, and manufacturers, and there is a ton of history behind them. I find them fascinating, both in how they work, and the history. There's a pretty large community of siren enthusiasts such as myself, and we often go out to record siren tests, acquire and restore retired sirens, and discuss them. It's been a hobby of mine for a decade now.

Street photography! Check out my post submission history.

Homebrewing. Idk it's niche, most people I talk to know about it, but hardly anyone does it. Judging by the size of YouTube subscribers on popular channels, it's less popular than 3d printing (which I also do).

Its not difficult, but can be tedious. If you can make oatmeal and wait a month, you can make beer. It's kinda like cooking. You can get good food at a restaurant, but you can make better at home though it takes more time. Beer is the same. Takes longer, but is more rewarding.

Soapmaking. My interest has faded a little in the past few years, but I taught myself how to make soap from just lye and oils. Maybe its more popular now? When I started out there was hardly anyone to talk to about it outside of a soapmaking forum. Maybe it's still kinda niche tho

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.