A pair of wooded former military islands east of Suomenlinna, opened to the public in 2016 after the army cleared the unexploded ordnance. The trails crisscross overgrown fortifications, gun batteries, barrack ruins, and a remarkable variety of butterfly habitats. Several rare species shelter here that exist nowhere else in southern Finland.
Ferries run from Market Square in summer (May to September). Pack water and a picnic — there is one small café — and good shoes. Allow at least half a day. The Helsinki Biennial occupies the island in odd-numbered years; the rest of the time it is simply one of the best nature walks anywhere on the city's doorstep.