Ill start off the thread you guys keep it going!
Some things I do off the board to be better on the board are,
Juggling (oddly enough spacial reasoning, or just the concentration has helped me a lot on the board).
Drawing (this has helped me put things into perspective)
Studying the way our brains LEARN, and attempting to apply the things i learn to my chess studies.
Meditation(this has helped me to clear my mind of any non-chess-random thoughts that used to waste some of my 'thinking space')
I think that if you keep yourself busy with some activities that require concentration that's nice for your chess in general. Reading, maths, languages,...
Getting to understand your emotions and how to control them in hard scenarios is something that can improve your chess, meditation works for this in my experience, stepping into situations that get you out of your comfort zone can also improve your confidence.
A lot of players would say that working out can improve your stamina in long tournaments.
Learning how to learn is something really important as you said.
Getting some regular physical exercise. I've started doing some basic body weight fitness routines a couple times a week and the impact on my mental ability in chess and at work has been pretty dramatic.
nice, adding another one - Philosophy and literatures which are related to battle or similar skills which make you stronger in one way or another.
Two examples: The art of war , great 'war' resource, however some of the literature can be converted to chess advise simply by changing the word 'war' to 'chess' 'in chess, entice your opponents with baits' etc.
also meditations by Marcus Aurelius - great audiobook on youtube ~ has made me stronger in general, which has helped when playing chess because i feel . . . stronger.
@custommods I often read this kind of books, not necessarily because they improve my chess but just because I like the topics, a book that I've found really interesting in terms of philosophical value and with a lot of similarities to chess is the "Tao Te Ching" by Lao Tzu, you might wanna take a look.
Greetings, Paul.
spinning a pawn while playing :P
@Paulcarrero Thanks I will check it out.