Keep Your Brain Alive, by Lawrence Katz and Manning Rubin, is a fun little book of easy things to do to keep your brain happy and healthy. I like how it takes the view (supported by scientific-looking footnotes) that brain cells are not irretrievably lost as you age. Rather, you can keep your brain actively growing throughout life, if you give it the right kind of stuff to work with.
"Neurobic" exercises aren't about doing mental puzzles, like Sudoku or whatever, though those are good, too. They focus more on the act of learning, since that's what gets neurons to forge more connections and stay healthier. But since we don't all have the time to go out and be a full-time student, these exercises break learning down into its most basic elements: associations. You can play around with lots of associations in your daily life with very little time commitment, and it'll help keep your brain energized and happy.
To make new associations, try finding things that use different parts of your brain and connecting them. Writing or brushing your teeth with your non-dominant hand is a good one. (My right hand actually twitches, trying to get in on the action, when I write with my left, which shows how strong an association I'm working against.) Mixing and matching senses is another handy way to find neurobic things to do. If you try showering with your eyes closed, you'll start adding new little wires in your brain connecting your sense of touch to the process of finding the shampoo, which had previously just been a visual task. If you always start your day with the smell of coffee, spend a week with something else to get you going, preferably something with an equally strong smell or taste.
Another key concept is to find mindless routines and change them up so they engage your attention again. For example, after a few weeks at a new job, you probably stop noticing your commute. So experiment with different routes, or different modes of transportation for a while. If you buy the same things from the supermarket every week, go to a farmer's market instead, and decide on-the-fly what to get. Any thing you can do to turn something "mindless" into something "mindful" helps.
I want to give props to the illustrator, David Suter, here, too. The book is full of his great little drawings that play with visual similarities and connections in a lot of really interesting ways. The perfect style for the subject matter.