For particular brain functions (learning guitar, coding, speaking another languages) actual practice. (This is also how training exercises work, only their particular function they improve the brain on is solving them, which makes them useless).

In general, better physical workout and diet does genuinely improve brain function.

Hard for the brain to function well in a 500lbs diabetic body...

Physical exercise. I'm not kidding. I recall hearing about a researcher that worked with people age (65+) to see how they could recover and/or improve their cognitive skills. He tried other things (including brain training) before jumping to exercise. Its research lab ended up being a Gym.

Brain training does work for some people; around 20% of the population won't gain anything from it, regardless of what you make em do. And even for those that works it has limited scope on the benefits.

Meditation works. We've known about the usefulness of it for thousands of years, and more recently there is a growing body of scientific proof [1].

Everyone really should try it. I firmly believe there is nothing that you can do in 20 mins a day that will improve your life more.

[1] https://en.wikipedia.org/wiki/Research_on_meditation

I remember reading studies about a few different video games improving cognitive performance: Rise of Nations, Starcraft, and an FPS (maybe Unreal Tournament?).

Actually learn new stuff? Learn a second language, an instrument, math, programming—anything that takes you out of your comfort zone.