The cape town of Hanko borders on three sides on the sea and is Finland's most southern town. Over 30 kilometres of sandy beaches and beautiful old villas (once built for visiting royals from Russia and where you can stay for the night yourself now) will probably make you stay for a few days- if not only to feed your hunger for spotting birds. There are some fifteen migratory birds points and several nature parks that offer excellent opportunities to spot all sorts of birds. In the town itself, you can either visit the ramparts of the 18th century fort or simply enjoy the excellent views of the sea. Although Hanko is a busy port these days, it was able to keep the atmosphere of earlier days.
Try to visit the nearby Uddskatan nature reserve (guided tour strongly advised) and follow the nature track Högholmen that leads up to astonishing views of open sea.