There are many of them. A vivid, popular example is the study of languages. This method is useful, interesting and opens up great opportunities. You will be able to safely travel abroad, read works in the original and advance in the sense of a career. Even helping your own children learn is a big plus.
There are many courses, all kinds of trainings and teachers, native speakers. The ability to speak a foreign language will develop your imagination, help you express yourself more competently. Travel. It does not work far, visit neighboring cities. You will be able to learn about the culture of other nations, religion, way of life. Comprehensive development and rest from everyday affairs will benefit.
Self-development of a person lies entirely on himself, as well as the construction of life. The desire to develop and not stop learning is necessary constantly. After all, there is no limit to perfection. The main thing is for a person to feel full, deep, spiritual personality and then the environment will perceive you the same way.