The Fjord Boats in Brevik trafic the islands Sandøya, Bjørkøya and Siktesøya in the archipelago of Brevik.
It is also recommended to take 1 hour round trip that runs from Brevik to Sandøya-Bjørkøya-Siktesøya and back again to Brevik.
Spend a day at one of the islands and take the boat back later in the day.
From the pier at Siktesøya there are forest trails to local open space.
Or travel to Sandøya and combine your day with hiking and swimming or cross the island on hikingtracks to Dikkon Beach. From here take the ferrie back to Brevik.
The possibilities are many.
Timetable for the ferries you´ll find here