A few I can list off the top of my head, in no particular order:
Assorted books/movies/TV shows - When I get into fiction, I get REALLY into it. Trivia, information about the actors, analyzing the characters, writing fanfiction, researching any relevant topics (ship diagrams for Pirates of the Caribbean, space travel for Star Wars, etc.), other works by the same writers, and so forth. I tend to go through these in phases, with some lasting weeks and others spanning years.
Psychology - Especially stuff relating to personality, but I've been known to spend hours reading on just about any aspect of psychology that catches my eye.
Weather - I check my phone's weather app 10-20 times per day, watch the radar to see if I agree with the forecast, track incoming storms, etc. I live in Texas, so tornado season is particularly exciting for me. The last time we had one go by, my mom had to pull me inside because I was intent on filming it. One of my bucket list items is to go storm chasing.
Autism - Somewhat ironically, I can monologue about ASDs for a solid 45 minutes before I'll realize what I'm doing. I work a lot with kids/teens who are on the spectrum and am in the process of writing my honors thesis on employing adults with AS, so it's definitely a topic I feel passionate about. Unfortunately, not all of my friends/family/casual acquaintances agree (or at least, not after the first 15 minutes).
Assorted skills - Every so often, I'll randomly decide to pick up a new hobby and throw myself into entirely for a few months, then move on to the next. My main two right now are baking and knitting.