The best thing I ever did to preserve and improve my brain was find things, really hard things, about which I was passionate. It's not so much about studying as it is about finding subjects and venues of learning that just kick your ass. If you don't have the experience of getting your intellectual butt kicked, and if you don't have the ability to learn from the experience, you can only grow so much.
The easy stuff shouldn't be avoided; you should just expect yourself to pick it up as necessary. If you are not getting challenged, if you don't encounter people who are way ahead of you in some subjects, then you are doing it wrong. The feedback is key. Getting stopped in your tracks, just when you thought you were running along at a good clip is invaluable.
Not that I ever achieved greatness, and I do sometimes ask myself why, even in my private life, I always pick the really hard to study (and have for decades now). I'm sure masochism is involved, but the good stuff is just hard, and you have to be willing to be someone's idiot some of the time to learn.