1) Fixing my worsening health symptoms which started almost 3 years ago. It had gotten to a point where I could no longer think or be productive for the past 6 months. After spending every waking moment researching every ailment that matched my symptoms, I eventually discovered I had Hashimoto's and had been hypothyroid for a really long time. This was after making the rounds with several specialists who had no idea what was wrong with me. My recommendation is to take charge of your health and be the best expert of your body. Being healthy is extremely important not just for the obvious reasons, but because everything stems from it. Don't take your health for granted, ever.
2) Learning German which taught me important memorization techniques to carry over to other areas of my life.
3) Stretching & flexibility exercises. I've been a heavy weightlifter ever since college but only began to realize recently that lifting weights can have detrimental effects on your body if that's the only thing you do. You'll become less flexible and more rigid which makes you more prone to injury. Especially with our profession where we're propped on a chair for most of the day, it's important to make sure our bodies can achieve a full range of motion to prevent any skeletal or joint issues down the road.
4) Cooking and eating a ketogenic diet. I'm much more satiated and rarely go hungry throughout the day. Cooking was extremely daunting at first but not so much when you really start doing it. It's an essential skill to have, ensures you're eating whole foods and putting nothing processed in your body, and gets you invites to more potlucks.
5) Writing everything down. My Evernote has exploded within the last year. I keep everything there and it's saved my ass countless times. You'll thank yourself later when you get into the habit of writing everything down.
That's all I can think of right now. What's exciting to me after having wrote this is that all these things were realized only within the last year or so. It's fascinating how much we can improve in such a short amount of time as long as we are consistent with always trying to grow and learn.