Meditation is one of the first things you run into if you start searching healthy mind on google or elsewhere. I will work to find different types of meditation, different teachers, different philosophies. Everyone can use some grounding to help them focus and find themselves.
In order to have a healthy mind you need to speak the truth to yourself. How many of us go through life without ever examining the things we say to ourselves on a daily basis? What does your self talk sound like? Are there lies that have become ingrained? We will learn to explore that.
Why would we need positive affirmations? We need to constantly bombard our minds with positive statements about the way we want our lives to look. Why? Because negative statements have much more impact and if we are not deliberate about building the positive, then the negative will win out.
Read! Read! Read! I cannot say it enough. Your mind cannot grow and stay healthy if you are not giving it new material to work with. Fiction, non-fiction, self-help, DIY, biographies, histories, etc.... Devour books on a regular basis.
Working a puzzle can stretch the mind. All muscles need exercise, and so does your brain. Keep it active, challenge it, grow it. If it is not growing, then chances are it is shrinking.
Learn a new language, learn about another culture, research a subject you are interested in, listen to new music, learn and practice a new word, etc..... Our minds have a great capacity for learning. Use it!