I figure that with increasing age, it's probably a good idea to keep the old noggin active, and that probably means learning new things. Sudoku is all very well, but once you've learnt the tricks, it's just formulaic.
I spent some time, not long ago, looking at resurrecting my old programming skills, and I wrote a small app for Android devices, using Java, a programming language I had not used before, and an app development environment from Google. This presented some challenges, not least being the steep learning curve. The lack of a teacher was mitigated by various support forums on the web (didn't have that when I were a lad) but in the end, insights were won at great cost of time and effort.
At the same time, I found I was running down the same lanes I had explored when younger. I don't find programming inherently satisfying. It's good to finish a program, but the process of coding I don't especially enjoy. It's a good feeling to be good at something, but better is to enjoy doing it, even if, in the end, you're not so good at it as you would like.
When I was I kid, I played piano. An aspect that learning process that I just hated was music theory. I considered it a necessary evil, and did as little as I could get away with. Bad call. Just as an ability to add, subtract multiply and divide, plus a knowledge of the times tables are gateways to the worlds of mathematics, so basic music theory is a gateway to harmony, arrangement and composition. You can of course create music without this knowledge, "by ear", but it's a lot easier if you have it.
I am learning music theory, harmony, arrangement and composition. I have a good teacher at the conservatoire at Evron, and I'm getting into it.