1. Read lots of books
Especially reading different kinds of books will improve your brain.
2. Do meditation
It will lessen your stress levels and make you focus better.
3. Solve puzzles
It's just like doing sport and having a fit body, but this time you have a fit brain.
4. Do sports
It will improve your circulating and respiratory system. And this will effect your brain too.
5. Learn a new language
It's both good for your brain and mental health.
6. Keep a diary
It will improve your creativity, imagination, and grammar.