thedæmon (nprofile…mzyr) Teaching and mentoring younger people. A lot of old people I admire are kept sharp by teaching young people the things they know, but also the things that are "current". The questions hurled at you from promising young people, will force you to contend with what you know, and try to integrate it into modern times, thus, finding essential wisdom threads linking the old and the new. You will likely find yourself also learning the new things, and building new things, with the people, or for the people, you're mentoring or teaching.
You mentioned physical activity, but also, do not underestimate the effects of diet on the brain. My wife and I have done a lot of experiments over the past few years, and while I don't claim any truths found from the few anecdotes I have, I can only say for near certainty that the effects on the mind, over time, usually months, seem to be major. I recommend approaching the matter with a printf() debugging mindset, minimize inputs through fasting and diet limits, usually starting with some ketogenic (vegan or otherwise) setup, then add things one at a time, ideally unprocessed, wait a week, add another thing, see how things affect you.
Reading is good, but writing is gooder :) Writing about things, or writing what you think, or writing a journal, or writing letters. A plus if you write with pen in hand, helps practice your hands' dexterity. I recommend writing for other people, over writing just for yourself, as the former sharpens the mind more, but a balance of both is good.
Something to ground the mind. We can work the mind in the abstract realm as much as we want, but as we grow older, I feel like the mind becomes more and more "floaty", and I fear it slowly gets "untethered", and that doesn't feel right. Presence, grounding the mind in the moment, in this world, feels important to me. Practices that connect the mind consciously with the movement of the body seem really good for that, like walking meditation, or some yoga-like stuff, or some very light martial arts, or archery. Archery is a good one, but any other "hitting the target" exercise through coordination between the mind and physical movement would probably work.
Sitting with one's own mind, for an hour or so, every day or few days, just us, my mind and I, for an extended time, feels really good too. A garbage collection process of sorts. Calming the mind waves, whether with an intentional meditation practice for half an hour, or just with a cup of hot tea in the park, listening to nature.
And a lot of that can't happen without: Forgiveness. We don't realize how much burdens our minds carry until we let go of the weight, how subconsciously distracting our background processes are, until we put them to rest. It takes a lot of mental work, it can be a bit painful sometimes, but letting go, forgiving, is the true freedom of the mind. Our time is limited, all of us, we won't last; everything else becomes trivia when compared to that.
Oh, and speaking of limited time, don't forget to enjoy some of it with friends and family. It's been a long journey with them perhaps, but it never is long enough. This is definitely one of those "has massive effects on the mind" but "no one really knows what to do about it" things.
But anyway, I'm rambling now, what do I really know~
I wish you the best on your journey of the mind :)