And whatever NEW challenges you feed your brain must be enjoyable and be challenges that hit on a lot of levels. I took up the fiddle, which is one of the hardest instruments to learn. Right Brain, Left Brain, Dexterity and Body Memory are all challenged on a regular basis. If I keep it going, I just might achieve competency at this infernal instrument right around the time I hit 70. I should also mention that it is addicting, and a great way to get even with the neighbors for their loud rap music. :-) It seems also that there are a whole lotta OLD fiddlers around who are still raising hell.
Whatever you choose to be your new "Brain Food" make sure it challenges you on similar levels. If not music, some sort of art, or craft that involves both intellectual skills as well as physical/coordinational ones is what you need to find. Whether it be rebuilding a classic car from the ground up, or furniture making, or refurbishing vintage audio equipment . . . whatever it is, it's got to be something interesting to you as well as challenging, and that when you start to achieve some competency, there are always going to be new levels to reach for.