I don't know that this even counts, but one of the most strange but wildly interesting things I used to do years back was randomly exploring defunct teleporters in Habbo Hotel.

For those who don't know about teleporters/teles in Habbo Hotel, there are probably tens of thousands of pairs of teleports that exist in the game, each of them connecting only to its pair. Since trading furniture is pretty much a currency in Habbo, a lot of individual teleporters get traded off or lost throughout the years, and often end up being parked in random rooms and vast furniture junkyards.

So I would often lay down several random teles from my inventory, or enter my own furniture junkyard, and try every tele in there until I got a live one. This would Bill & Ted me to fuck knows where. If I'm unlucky, it's just a dead end room. If I'm lucky, it's a room with even more teles. That's where the rabbit hole begins. Pretty soon you're ten teles deep into the weirdest, most liminal Back Rooms spaces you can imagine. Sometimes you even find a back door into other players' private rooms and get to explore like a cat burglar. The sky was the limit.

I haven't logged in for a decade or more, but I still miss doing that sometimes.

I included the best pic I could find online of what a tele goldmine looked like, except there would typically be a wide variety of styles and not all portapotties like these.

In Eve Online, when a capsule was destroyed, a frozen corpse was left behind.

I knew someone who would go around collecting corpses. A battle is going, he'd be out there scooping them up. He's running a hauler, and this was the day that when your ship got destroyed, every bit of loot went out in individual units, so when a pirate would try to shake him down he'd respond with "If you blow me up, you'll crash back to desktop."

That was how he played the game, gathering corpses.

Memorising door colours of every house and flat in a small Scottish town of about 30,000.

Yes, the hobby-ist was on the spectrum.

Anvil firing

You get 2 anvils

Pack some gunpowder between them

Light a fuze

Run

The top one shoots off into the air

And you try not to looney tunes yourself.

I'm surprised no one has mentioned hobby horsing yet.

For a bit of maybe context, she was a paleontologist...but

Friend of mine from university was always ready to scoop up roadkill into her trunk when she passed it by so that she could render it down to the bones so she would have a skeleton to study/draw.

If there's one thing I learned from being a siren enthusiast, it's that if it exists, there's a community and hobby formed around it. Neurodivergence is a helluva drug.

Counting counties aka keeping track of every county you visit. My husband has this hobby, and sometimes on road trips we drive way out of the way for him to grab a new county.

Classifying plastic bread bag clips

I dunno if it's really a hobby, but one time I heard about bug collectors. They're not people who go around catching insects, instead they're people who go around catching STDs. On purpose.

That I've heard of? I'm gonna count that guy who commissioned fanart of women buying way too much Wonderbread and destroying rainforests.

Thinking about that guy, he's probably having the time of his life with AI image generators.

The fire alarm collecting community is larger than you might expect. I first got one recommended to me on New Year's of a guy setting off all 20 or so of his linked fire alarms when the clock struck midnight. He needed hearing protection the next room over.

One time I was at a bonfire and a friend of a friend, looking like he was up all night, said he was up all night watching tornado siren videos on youtube

I used to raise mystery snails and sell them on eBay and Aquabid. Margins were too thin. After shipping in their special bags, and worse if I had to add a hot or cold pack, only netted me about $5 a sale. Still, never met another that did that for kicks and the same couple of sellers were still active after 10 years last I checked Aquabid.

I don't remember the name, but the being hanged by chains attached to your skin thing is really weird

Trainspotting (a now largely obsolete English hobby). Back when trains were a thing, nerds would gather at at the front end of the platform to write down the numbers of the engines. Exciting stuff. If you ever saw a movie called 'The Station Agent' with Peter Dinklage you kind of get the idea. Now a slang for a pointless, useless activity (which is where the movie about Scottish heroin addicts gets its name from).

A close second is commenting on the internet.

The telephone pole appreciation society...

https://www.telegraphpoleappreciationsociety.org/