Playing the piano, initial thoughts
I rather enjoy making my brain twist and turn, to make it work, to give it the muscle shock, and say: "Hey! Dummy! Wake up! It's time for you to get back to growing."
Off the top of my head, I would say learning a new language, learning bridge or chess, and now, playing the piano.
As a creature of habit, and shallow habit at that, there are very few activities that that require such deep and deliberate thinking as to cause me to question whether my brain is actually working at all!. And one should never miss out on an opportunity exhaust the brain!
Over the past few years I have picked up some new hobbies. Snowboarding required that I give up my old ways of balance and learn some new tricks. But all in all it is fairly shallow brain work. After a few weeks, riding switch went from brain to body, the mechanics of the rail slide stopped passing through the brainputer and went straight from eyes to legs as far as the information pipeline is concerned.
But now, this piano. The idea of making the left and right hand work together is a concern that I never considered would be so difficult. And I don't even want to think about the pedal. My mother's piano had three pedals. But then there is the reading of the music. And now I find out that once I am comfortable with my hands resting in C major, I need to figure out how to move my hands up a few notes and shift the entire context of the finger-to-key mapping in my brain. It's unbelievably taxing and rewarding. I am unsure of any activity so demanding of the brain and body.