Honestly, with most of my interests I can only really discuss them with one or two people IRL. Otherwise people either don't give a shit or mock me to the point I won't mention it around them.
I really like true crime, especially that pertaining to female killers. Most of my family has always bitched about how I am rotting my brain with violent garbage or joking while I'm watching them that I'm learning how to kill them in their sleep by watching these shows, so I just never watch them with other people. I never gave them shit while they watched Disney Channel or Hallmark movies. I probably should've. Most true crime fans also tend to be quite nuts (Stephanie Cianfriglia is an extreme example).
Trashy TV in general. I have a couple friends and my partner where I can watch and laugh at this shit with, but same deal. My fiancee actually got very offended when my mom "warned" her after they first met about how I've been watching daytime talk show garbage since elementary school, when the likes of Povich and Jerry Springer would come on right after Pokemon. Considering how much I studied in school, maybe I want to laugh at lie detector shows and Judge Judy verbally tear apart a scammer between hours of homework?
Bande desinee outside of Europe
Donald Duck outside of Europe
McDonald's. Its history, marketing (especially the gaffes and scandals), and localized offerings fascinate me. McDonald's also used to have a lot more unique buildings and varied interior decor compared to other fast food joints (e.g. that UFO at Roswell, McBarge, full blown Cabbage Patch Kid doll dioramas at the location by where Cabbage Patch Kids were created). But it's also so "lol fat" that I don't like talking about it in person. Or even online, really.
I'm not as big into anime and manga as I was as a teen, but it sucks that I don't really have anyone outside of my partner or a couple old friends to discuss my old favorites with. Not to mention most otaku nowadays are trannies that are somehow bigger deviants than the otaku I went to school with. I have not been to a convention in years and I'm (mostly) fine with that.
Nail polish. Not just brands and colors but also the making of it, hand care, nail anatomy, etc. The few people that ask basically nope out once I sperg or are like "k"