I don’t know how niche they are, but a few I’ve done in the past

- 3D printed musical instruments. Print other designs or contribute your own

- lock picking. When you really get into it, you modify locks to make them more of a challenge and mail them to people

- Ham radio is hundreds of sub-hobbies in a trench coat. I’m currently mainly interested in linearizing switch-mode amplifiers, but was doing fox hunting for a bit (radio direction finding), and periodically do POTA (transmitting from parks)