There are links to several specific brain exercises below. I think these exercises are excellent, but there are many other ways we can work our brain. Some require serious time commitments, but others can be integrated into your daily activities with virtually no extra time spent.
Integrate exercises into your every-day activities; do saccades between any two points while waiting in line at the store; estimate the total cost of all items in your shopping cart as you put them into the cart; switch hands with your knife and fork when you eat; brush your teeth with your opposite hand; use scissors with your opposite hand; put on your shoes and socks while standing and balancing on one foot; try recognizing different spices by smell alone; count backwards from 100 by sevens while waiting at a stoplight (but not while you’re driving).
Start new activities: dancing; bowling; skiing; tennis; climbing; swimming; ping pong – anything that requires learning new skills. The older we get, the more important this becomes.
Learn new skills: study vocabulary; learn a foreign language; learn to play an instrument; memorize your drivers license #, credit card #, or your family’s phone numbers; do math and logic puzzles. Work on your weaker areas. If you’re great in vocabulary and bad in math, do math puzzles, not vocabulary.