Could be boats. There are a lot of jokes about that too. But there are a lot of hobbies that can just suck in crazy amounts of money. MTG cards, Warhammer figurines, model train sets, boats, consoles and games, anime/movies/series/manga (I have a friend who has entire walls filled with this stuff). Anything were you end up collecting stuff, especially things which has gained value over time because of rarity and interest from collectors.
"A boat is a hole in the water into which one throws money."
Sadly, it's true. Worth it to me -- for the sublime life experiences I think my family and I wouldn't have had any other way -- but it makes no fiscal sense.
My father has a sailing boat which needs maintenance like a house and a car, since you can live on it and it has motors, except you're fighting against corrosion and water all the time. He does a fair share himself but crucial stuff (hull, sealings, ...) is safer when done by pros. And equipment is outrageously expensive : Sails ? 10k. Desalination machine ? 15k and so on...
If you're DIYing boats are cheaper than cars. You'll never be tempted to buy chromo axle shafts and coilovers for your boat and a hull is a hull you either work with it or get a different one. The expenses of setting yourself up to DIY everything are also much less since you won't be temped to build out a fairly well equipped fabrication shop. One of my best friends does boats. He spends way less money on it than me.
> Anything were you end up collecting stuff, especially things which has gained value over time because of rarity and interest from collectors.
At this point questions might start coming back. Just hope your hobby won't be destroyed by speculators who don't care about it, but see an opportunity to see it like a different stock market.
And if not, do you mind sharing what you do?