For me it was strength training with a good quality personal trainer. It cost me 1k in total over a few months, but it changed my life completely. My trainer not only focused on the workout, but always took a holistic view of my life, including mental health. I still live by his words of wisdom and am immensely grateful for him turning my life around. Best 1k I ever spent.
I'm interested if others have any such stories to share.