Hmm, maybe a microphone collection?
There’s all these interesting concepts and ideas involved with microphones. The fact that a physical thing imparts some kind of character to the sound is really cool. Some work well with percussion, some work well with vocals, etc. Also, it kind of blew my mind that you can do mid/side capture with microphones too.
Hah, these things probably would seem kinda obvious to people coming from a less electronic direction (?)
But I wouldn’t be getting anywhere with my music, the occasional field recording or some specific sampling is all I ever really get in contact with microphones.