A small island in the harbour halfway between the city and Suomenlinna, formerly used as a Russian mine-degaussing station. Since 2014 the buildings have been gently restored and opened to the public — a sauna, a single restaurant, a few small exhibitions about the island's history.
Lonna takes about an hour to explore. The ferry runs from Market Square between May and September, every couple of hours; bring a swimsuit for the sauna, which has open sessions on most afternoons. Quieter and far less crowded than Suomenlinna.