Change your routine. Take a different route to work, try a new recipe for dinner, or get involved in a challenging activity like ballroom dancing. New activities keep your brain cells guessing. Studies have shown that stimulated brain cells generate new cells.
Do crossword puzzles, sudoku or word problems. Don't get discouraged if you get stuck on a problem. Brain scans have shown that when you get stuck on a problem, it helps your brain grow because it has to try harder.
Get engaged in a conversation or debate, do more in-depth searching online, or try anything that requires you to make learn in order to make an informed decision.
Exercise. Not only does exercise produce endorphins, which make you feel good, but it also helps keep your synapses stable. Dr. John Ratey, a clinical associate professor of psychiatry, told MSNBC: "Exercise in many ways optimizes your brain to learn." Both cardio and weightlifting are great choices.
Learn a new language or musical instrument. These can be particularly good challenges as you get older.
Travel to unknown places. You're more relaxed at home, and traveling pushes you to be more aware. You also encounter new people and foods, and learn about interesting cultures and their history. Traveling also kicks in your survival skills.
Attend a class or lecture on a subject you're are interested in. Sign up for school and finish a degree. It will give you a sense of accomplishment and stimulate your brain at the same time.