Själö, Seili in Finnish, is the most memorable side trip from Nagu. The island is green and quiet now, but its history is severe: it was used first for a leper colony and later as a mental hospital, cut off enough by water to serve as a place of exile.
The old wooden church remains, and the University of Turku now runs the Archipelago Research Institute on the island. In summer, boat connections make Själö a realistic day trip from Kyrkbacken or Turku; outside the season, check transport carefully before planning around it.