Intellectual stagnation is a horrible thing. It happens even to very smart people, because basically a smart person, like anyone else, will gravitate toward the already known, as a matter of comfort or inertia. If you comfortably ensconced in what you know, there will be few challenges.
How do you know if you've stagnated? If your ideas five years ago were the same. The jokes you tell. Are your courses identical? Do you cook the exact same dishes? Wear the same kind of clothes?
I've decided the best thing is to learn another language, where language means any cognitive system that must be approached with some degree of rigor. So music would count, or a branch of mathematics, or an actual language like Greek. It could be drawing or ceramics, or chess. It should be something you learn to do, not something you learn about. The point is not the accumulation of additional information. Nothing wrong with that either.
I am incapable of having a hobby without it changing my life in some profound way.