Llyn Clywedog is a man-made reservoir north-west of Llanidloes, held back by the tallest concrete dam in Britain. The engineering is part of the appeal, but the drive around the water is the real pleasure: steep wooded sides, wide sky, and sudden views across the lake.
Stop at the dam and viewpoints, then look for Bryntail Lead Mine below the wall if you want a short historical detour. The reservoir also has osprey habitat, fishing, and dark, quiet edges that feel much further from town than the mileage suggests.