Hitting yourself on the head with a rock - Works on important physical skills, similar benefits to martial arts. Helps you overcome limitations. Reinforces actual reality of macro-scale objects, even when quantum mechanics is true. Teaches you about the difference between good and bad ideas.

From what I understand, the way that people get good at chess is by endlessly honing their ability to recognize a wide variety of visual patterns that happen to recur in the game (and especially at the higher levels, to some extent by just memorizing trees of opening moves), rather than by understanding deep principles or learning deep skills that generalize to other domains. As with anything complex that many smart people have spent a long time thinking about, it has a lot of interesting little details in its culture, and as with anything that takes sustained and concentrated effort and punishes mistakes, it probably teaches qualities like persistence and self-control, but on the whole I would definitely recommend against trying to become good at it if you're not having a lot of fun.

This sounds like a list of 'any nerd hobby' (with the honorable exception of 1 out of the 6, poker).

Improv theater -- Practice in examining and altering your motivations. Sharper-than-usual distinction between terminal goals (humor, drama) and immediate goals (those of your character).

Game design -- Similar benefits to chess. You're in a situation of perfect information and clearly-defined rules, and yet you still bump very quickly against the limits of your own cognition. You're forced to acknowledge and try to work around your limitations, because there are no excuses. Unlike in chess, there are no unsolvable problems; at worst, there are problems with no elegant solution.

Writing pages on the tvtropes wiki -- Heavy overlap with the goals of lesswrong; there are even tvtropes pages for fallacies and biases. Learning to spot patterns in literature has the same challenge as learning to spot them in thought: the most omnipresent ones are also the least visible.

Anything to do with language -- Translation, conlangs, writing in English with arbitrary restrictions...they all help the same way that Rationalist Taboo does.

Wei_Dai:

Science fiction - Reduces status quo bias and gives interesting insights. Also teaches that the way a society is organized depends a lot on the set of technologies it has access to, so if you don't like how your society works, one lever you have is to change that set.

From what I've seen of it, I disagree. With a very few exceptions, even the highest-ranking SF in terms of popularity and critical acclaim is usually badly written, and instead of exploring truly imaginable and inventive developments, it projects the prejudices and illusions of its own time embodied by the author.

I'd be very curious to hear about some counterexamples from other commenters, though.

Just read through the comments. Ouch. As Nesov said in the absolute denial macro discussion, "this is turning into a Let's Spout Nonsense!!! thread", but at least that one didn't make me want to unsee. Cheers to Manfred.

Tabletop Roleplaying Games: Two main benefits:

Problem Solving and Lateral Thinking: With a good GM, you should be able to be challenged in ways that you can beat, but aren't obvious. Particularly in GURPS, it doesn't devolve into "I hit the monster until it dies". Ever thought about how you'd negotiate with terrorists? Escape imprisonment? Survive a zombie apocalypse as a vlogger newscrew? When you play in person, you're forced to think really fast. ("I can attack this which is doing the most damage, but it'd take a while and there's still lackeys", "Do I start healing this guy now, or just finish the fight?", "What can I possibly say to make this seem innocuous?"). Also teaches the importance of planning ahead ("What can I say to the guard so that X will agree with me, even though we can't talk?").

You also have rules to brush up against, and designing a good character is difficult. Do you try and remain a generalist? Specialize at something really useful? What's your role in the party?

Social: You have to play and deal with other people. You play the game in-person and it is prone to making a lot of in-jokes. Brings friends together and whatnot. It also has a lot of the same benefits as improv with regards to trying out new character traits. You're not supposed to be pretending to be yourself, and you can try out personality tweaks in a way that's not reflective of yourself. I played outgoing characters before I became less awkward as a person.

Musical performance. A few things I think are rationality-enhancing:

Realizing the difference between what your perception and others'--and between your perception and your recall. A confident, assured performance that has many mistakes you are aware of will often be impressive to an audience who is not expert or does not know the music--presentation goes a long way. But an expert will often perceive mistakes that you don't. And hearing yourself recorded will very sharply inform you of the difference between what you recall and what you actually did.

It's also useful for learning to understand others--especially playing in a section. (Orchestral winds usually play 2-4 to a section, with parts partially independent and partially in harmony; the first player is the "principal", while the 2nd and subsequent players generally attempt to match the tone and playing style of the principal so that when the parts harmonize, it sounds like a single player. Orchestral strings play in larger sections--8 or more--all playing the same part as the principal.) Learning to model and anticipate others' musical behavior--how they will react to conductors' gestures, to unfamiliar music--and imitate it. How will they think about the music; what do their physical gestures indicate that their future actions will be? Get it correct and be rewarded by making beautiful sounds and impressing those around you.

Self-improvement through imitating and analyzing successful techniques: when I want to play better, it often helps me to consciously imitate people who are better, and then am able to identify what it is I'm doing that is causing the improvement. If I want a richer wind sound, I'll try to imitate a college friend's style and mannerisms; in doing so, I realize, I'll sit straighter, take deeper breaths, relax more, be less hesitant at the beginnings of notes. If I want to be more agile in a tricky string passage, I'll try to imitate a different person, and realize I'm not tensing up as much, I'm using less bow motion, and moving my fingers faster.

Instant feedback: you can hear when you have or haven't mastered a task. You can try again continually until you reach success. (Also, not-so-instant feedback: will strangers will pay to hear you more than once?)

Emotional control under stress: performance can be very emotionally stressful. Learning to control this through practice is valuable. (In my first college recital, I had to stop halfway through because I was in a full panic: I'd forgotten the music and I was shaking too hard to continue. I left the stage to calm down before returning. I'd panic similarly when forced to give speeches in public. Through sheer repetition I'm now mostly unfazed by giving recitals or speeches in front of large crowds.)

No upper bound: I'm not going to exhaust the available material, as new challenges always exist and new ways to further refine existing skills are always possible.

And not a lesson but a benefit: status-raising (as some Overcoming Bias posts tickle the back of my brain). It has pretty much always been a plus in my personal and professional life to mention that I am a classical musician, particularly as many high-achieving people had some musical training. It makes people think both that I come from a higher social class than I do, and that I am more diligent and conscientious than I actually am...

The knowledge of when to get a new hobby when you've squeezed the juice out of your current hobby might be the master skill, though I supposed it can't count as a hobby in itself.

Sports - By watching sports and being a fan, you can learn about biases, first recognizing them in others and then in yourself. You can learn to evaluate strategies based on how well they help one win. By taking part in arguments and analysis, especially in more statistically sophisticated (sabermetric) communities, you can learn to reason based on data and evidence and to judge the quality of an argument. You can test yourself by making predictions and observing the results, especially if you gamble or play fantasy sports. (See also here and here.)

Politics and history - lets you see what answers you would give to difficult real-life problems, and you have to keep in mind that the people who dealt with them at the time were really extremely smart and politically able. They might not have been experts on rationality, but they were certainly experts at it. Learn from their failures.

I play video games because:

As for video games, the take-home message is clear: we should all be spending more money on better televisions, graphic cards, video game systems, and sound systems, and we should be encouraging girls to join in and reap the cognitive benefits.

• Do video games enhance cognitive abilities?
• Video Games Boost Brain Power, Multitasking Skills

Magic: the Gathering is good.