1. Finish a crossword puzzle. Just like a walk exercises your body, puzzles give your brain a workout. Think how satisfying it is to fill in that last clue.

2. Pick up a book. Whether it’s the latest novel by your favorite author, a new historical work, or a how-to book, reading keeps your brain active and nimble.

3. Try your hand at bridge. Or if you’re already a mean bridge player, keep it up. Card games provide ideal mental exercise as the brain ages.

4. Become a social butterfly....