I think continuously becoming a better programmer is probably a good way to go at this. If you can sprinkle in some traveling and learning another language or to while you're at it, that would perfect. And maybe learn a musical instrument as well.
Just went on a trip with my recently retired father.
The effect of a total change of routine, new experiences and challenges, emotional experiences (stress, wonder, happiness), was huge.
It was the first time I saw him look alive in months.
In retirement, I think you cam get stuck in am easy, comfortable rut. As in working age life but more so. Travel to novel places, with novel people, doing novel things, can force you out of that rut. Probably worth remembering for younger rutters as well.
There is so much novelty one encounters when traveling, on multiple levels. I definitely recommend everyone to travel and visit new places as much/often as possible, even in their own country.
Of course, I'm not talking about candy tripping in Ibiza.
That's my medium to long term plan, anyway.