Grianan of Aileach is a ringfort crowning a hill west of Derry, with broad views that explain why the site mattered long before modern borders. The reconstructed stone walls are impressive, but the panorama over Lough Foyle and the Donegal hills is just as memorable.
Although it lies across the border in County Donegal, it works naturally as a short excursion from Derry. Go in clear weather and bring a wind layer, since the exposed hilltop can feel much wilder than the distance from the city suggests.