Långholmen is the green island wedged under Västerbron, between Södermalm and Kungsholmen, and for a century it was where Stockholm kept its prisoners. The crown prison closed in the 1970s; the cell blocks are a hostel now, the rest is woodland, cliffs and small beaches. Three kilometres, a loop, best in the morning before the swimmers arrive.
Come up at Hornstull and cross Långholmsbron onto the island. The Kronohäktet — the prison, built 1874–1880 — is the anchor: corridors you can walk, a museum for the harder history, and the sobering fact that Sweden's last execution happened here in 1910.
Then it softens. Långholmsbadet, the sheltered south beach; the cliff paths over granite where the city briefly vanishes; and Västerbron, the 1935 arch, sweeping overhead — stand underneath where it lands and the scale of it lands too.
Follow Pålsundet, the narrow channel back toward Södermalm, with boats moored along it, and Pålsundsbron returns you to Hornstull.
