Richard Long, Heaven and Earth, at Tate Britain.
His work is inspired/created by his long walks, all around the world. He changes the landscape in some very small way, for example, by scuffing a line in the ground (as seen above), or by turning stones on their side, or placing them in lines or a circle, and photographs the result. It's a very simple idea, but quite effective, and quite beautiful. He also creates artworks out of maps onto which he's tracked his walks, as well as some indoor sculptures. Those are less effective, I think, but still quite nice. There were also walls painted in mud, and poems, consisting of not much more than lists of what he'd seen or where he'd been.
His website is here.