Some years ago I got back into math basics, so I could do mental math as a hobby. I'm still doing it today, nearly daily. I've also added crosswords. I've found

Some years ago I got back into math basics, so I could do mental math as a hobby. I'm still doing it today, nearly daily. I've also added crosswords. I've found that this helps to keep me sharp (I'm in my 40s), and should help me even more as the years go by.