Playing billiards and having tons and tons of books. I also forgot to add chess in particular.
People can possess enormous collections of books, and do, but I really bet them that they don’t pick up to read a single one. Only to sometimes bring up certain lines to win online and pointless arguments over.
Billiards is usually a bar/pub kind of game, but some others seem to associate it with intelligence. You don’t need to be intelligent to play billiards.
Chess is self-explanatory. Lots of people throw around the line of “people are playing checkers while X plays chess” and other similarities. I think people look way too into chess as being a measurement of intelligence.
How many people actually collect books for show? That seems uncommon and it should usually be fairly easy to tell for people who are somewhat well-read.
The thing with chess is that it’s not fun if you aren’t any good at it, and the difference between people who are somewhat good and those who aren’t is pretty big. You can get there with pure perseverance (same with most other things that gets listed here, probably), but most people tend to pick hobbies that they don’t have a hard time with.