In my opinion, training; specifically, core training is the best. I've been to several chiropractors, and what they do to benefit me vs the BS of the stuff they try to push on me isn't worth it.
Kettlebell training, stretching/yoga, calisthenics, and carido are essential. I guess it's obvious, but being in the best physical condition you can be in, and eating properly are essential. If you don't take care of yourself while you're in your 20's, it will be difficult (but not impossible) to alter that later on.
If I didn't take care of myself when I was in my 20s-eat right, focus on fitness as much as I did, I'd be a wreck now. I know it. My Primary Care physician even says as much.