Build a reading habit. Books have evolved as a central tenet of knowledge preservation of societies for a reason - they contain a lifetime of wisdom in a few hours of consumption. Read books that are 10+ years old, like High Output Management, The Mythical Man Month, and Good Strategy / Bad Strategy. They’re written by people who have done great things and condensed great lessons, don’t underestimate the depth of the words and revisit them often!
Make at least 2 really close friends that you can tell anything, preferably outside of work. You can make friends at work, just make sure you keep the friendship and hang out afterwards since work can distort relationships. As you get older it’s generally harder to make friends, but just as important to have them - so make them young!
Exercise is great, has been mentioned multiple times. Carve out an hour a week, start small, but be consistent. Healthy body = healthy mind!