The best way to exercise your mind is to do any type of problem that involves you to think. Simple answer right. So, if you want to keep your mind sharp, start writing, reviewing a few math problems, read a newspaper article and than summarize it, learn a new language, teach somebody something you learned, take up a new hobby, do something different, do things with your left hand if your right handed, and right hand if your left handed, you get the idea use your brain, and Challenge YOUR Life!