• Read/watch/listen more: Buy or borrow books, subscribe to interesting newspapers or journals, watch documentaries and listen to news or talk-shows to get regular dose of intellectual information.
• Play games: Try intellectually stimulating games or puzzles like Sudoku, Scrabble, Chess or Mahjong.  Do the crossword in the newspaper or do a jigsaw puzzle.
• Learn to appreciate the arts: Attend exhibitions, plays, musicals or concerts. Visit the library or a museum.
• Listen and engage in constructive debate: Share your opinions and thoughts on any issue of interest with a diverse group of people. Try viewing issues from another perspective.
• Take care of your body: Exercise regularly and adopt nutritional eating habits to maximize brain functioning as many nutrients are essential to good memory and concentration.
• Test your recall: Make a list — of grocery items, things to do, or anything else that comes to mind — and memorize it. An hour or so later, see how many items you can recall. Make items on the list as challenging as possible for the greatest mental stimulation.
• Draw a map from memory: After returning home from visiting a new place, try to draw a map of the area; repeat this exercise each time you visit a new location.
• Do math in your head: Figure out problems without the aid of pencil, paper, or computer; you can make this more difficult by walking at the same time.
• Challenge your taste buds: When eating, try to identify individual ingredients in your meal, including subtle herbs and spices.
• Take a cooking class: Learn a new way to cook. Cooking uses a number of senses: smell, touch, sight, and taste, which all use different parts of the brain.
• Create word pictures: Visualize the spelling of a word in your head, then try and think of any other words that begin (or end) with the same two letters.
• Learn a foreign language: The listening and hearing involved stimulates the brain.
• Let the music play: Learn to play a musical instrument or study music.
• Refine your hand-eye abilities: Learn a new skill that involves fine-motor skills, such as knitting, drawing, painting, assembling a puzzle, etc.
• Engage your senses: Try activities that involve as many of your senses as possible, such as gardening.
• Learn a new sport: Take up an athletic exercise that utilizes the mind and body, like golf or basketball.
• Remain socially active: Social activity not only makes physical and mental activity more enjoyable, but it can also reduce stress levels. Stay connected with friends or plan group outings.

Reference:
http://www.smrt.com.sg/STF/active_aging/index.html#intellectual
http://www.everydayhealth.com/longevity/mental-fitness/brain-exercises-for-memory.aspx?xid=nl_EverydayHealthHealthyAging_20081109