I do math (mostly geometry and trig at the moment) in an effort to stay sharp. Doing math works like crossword puzzles but requires more thinking than just memory.
Here's the link to a fun math problem site for anyone interested...Premath.
The really difficult ones do cause a kind of brain pain. But it is those brain-pain problems that produce the greatest elation when finally solved.
I've always thought that this brain pain must be caused by forcing the mud in my brain to move around. It's a bit like stretching stiff muscles. It hurts initially but the pain goes away shortly...after my brain loosens up.
I’ve got a condition that causes extreme exhaustion and muscle fatigue- with that I also get brain fog- bad- hard for me to concentrate or figure out c9mplex problems anymore. Even the brain gets exhausted- not fun!