Out Stack is the last scrap of Britain, a bare islet lying beyond Muckle Flugga off the north end of Unst. You do not come here for facilities or even, usually, to land; its travel value is geographic and atmospheric, the final rock beyond Hermaness where the map runs out.
Most travellers experience Out Stack as part of the view from the Hermaness walk, with seabirds, cliffs, Muckle Flugga lighthouse, and the open North Atlantic doing the real work. Bring binoculars if the weather is clear, and accept that fog or wind may turn the northernmost point into an idea rather than a visible object.