@roser - I definitely think sudoku counts. I play it all the time as well, you might say I'm addicted! Thankfully it's one of the rare addictions that is actually healthy. Sudoku requires you to use concentration, logic and common sense so it kind of gives you a cognitive workout, you might say. The best thing is that it’s accessible anywhere; you can play it on your phone or in the newspaper or on the Internet.
I guess you could make a comparison to physical exercise; just like physical exercise keeps you physically healthy and prevents muscle loss, mental exercise like sodoku keeps brain cells from dying and promotes better brain function.