Walking
About six months ago, I started walking every day. Most days I walk for about an hour.
I did not start for health reasons. I already do regular cardio workouts and lift weights. That said, I had become convinced prior to starting to walk regularly that it has health benefits independent of other exercise. And, indeed, my knee and hip joints feel a lot better.
My main motivation was the potential mental benefits of a distraction-free chance to think clearly for a sustained period of time. I had seen lots of anecdotal evidence online that this would work well.
It works really well. I now consider it one of my most productive hours every day. I tend to go first thing in the morning, prior to doing any writing. I find that I often work out and organize entire pieces of writing in my head on the walk.
I do not yet have a strong opinion on whether walking helps primarily by allowing you to think more clearly or simply because it eliminates the distractions. It's obviously both on the margin.