To stay mentally active, commit yourself to the idea of lifelong learning, because the key is to add novelty to your life by learning and doing new things. Great ways to stay mentally active include reading and writing, doing crossword puzzles, playing games, attending lectures and plays, and taking up a new hobby. Even watching TV can be helpful, but only if you're watching it to learn something new (for example, to learn how to build a workbench) instead of watching TV simply to pass the time.
Sources:
Brain health. Alzheimer's Association. October 18, 2007. http://www.alz.org/we_can_help_brain_health_maintain_your_brain.asp
Miracle-Gro for your mind. Crook, T. December 2007. Prevention, 124-125.
Shankle, W. R., & Amen, D. G. 2004 Preventing Alzheimer's: Ways to help prevent, delay, detect, and even halt Alzheimer's disease and other forms of memory loss. New York: G. P. Putnam's Sons.

