Horror movies, video games (mostly retro RPGs and a couple FPS), musically I wrote and played in metal bands for 17 years, but for privacy reasons I prefer not to discuss that publicly, and I'm a buff for the following academic subjects as a hobby: comparative religion, mythology, anthropology, folklore, and theoretical astrophysics.
TL;DR -- I'm a massive nerd.
In my middle 20s, I found myself to be very financially poor, and I found reading articles on the internet and hanging out at the local libraries and flipping through books there to be a very cheap hobby. So in an effort to save money, I'd smoke a bowl or have a couple whiskey cocktails and launch myself down a Wikipedia rabbit hole, or slap on a documentary on youtube and just listen to it and then do cross-reference research on it later.
Cross-Referencing is a hobby of mine that started probably when I was 14, originally for an assignment in my high school English class. The assignment was pretty simple: Find two movies of a similar genre, and write a compare/contrast essay about them. So, I landed on Eternal Sunshine of the Spotless Mind and Vanilla Sky. I actually really enjoyed the assignment, and as a teen I'd never really thought of doing that before so I began to naturally do that with everything as a hobby.