Walk three times a week

In a year-long trial with older adults, this much walking grew the hippocampus, the brain's memory centre, by about two percent. The comparison group, who did stretching, lost a bit over one percent over the year. Evidence: decent Memory improved in both groups, so the walking can't take all the credit for that part. Evidence: decent One possible player is BDNF, a protein exercise raises in the blood. That it's the cause is plausible, not proven. Evidence: decent

That study was in people over 55. Younger brains are less studied, but the direction is the same, and the general link between aerobic exercise and brain health is strong.