Soderskar is a secluded lighthouse island in the Gulf of Finland, set in the outer Porvoo archipelago among low rocks and open sea. The old lighthouse is the obvious draw, but the setting matters just as much: the island sits in a bird reserve, so access is controlled during nesting season and usually works best as a planned boat trip.
Come for the combination of lighthouse history, sea views, and the feeling of being far out in the archipelago rather than for a quick city sight.