The answer: Exercise.
Interestingly enough, not all exercise yields the same results. Aerobic exercise is more effective than stretching and weight lifting. Most studies used walking, as this is the easiest and most accessible exercise for older adults. It was found that the cognitive improvement correlated to the distance walked and not to the speed. So now that you know, take a long peaceful walk, improve your mind and enjoy the view.
(Taken from Hertzog et all, Scientific American Mind, July-August 2009)