Here is one of mental exercises featured by everydayhealth.com that can keep your brain sharp:

Exercises to strengthen brain function should offer novelty and challenge — and they can be fun.

“Almost any silly suggestion can work,” says David Eagleman, PhD, neuroscientist and assistant professor at Baylor College of Medicine in Houston, Texas. “Drive home via a different route; brush your teeth with your opposite hand. The brain works through associations [which is why it’s easier to memorize lyrics to a song than it is to try and remember the same words without music], so the more senses you involve the better.”