Read books constantly. Read books that you enjoy, books by your favorite author or even books on subjects that are new to you. Reading is good for the brain and feeds it new content.
Go back to school to study a subject you always wanted to learn. This will increase your knowledge base and may even lead to a rewarding career down the road.
Watch the local news and world news daily. Stay informed on what is going on around you. Connect what you know about history to what is going on in the world today.
Do things that stimulate your mind. For example, do word puzzles, paint a picture or play a challenging game like chess. Even video games will help keep your brain sharp.
Learn a foreign language or learn to play a musical instrument. Such knowledge will allow you to expand your horizons even further by traveling abroad or joining a band.
Stay open-minded to learning new things from people. Don't be afraid to open up to new cultures, ideas or outlooks on life. Watch a different genre of movies or listen to alternative music genres. Your tastes may change and expand.