Shiraishi Island is the best reason to put Kasaoka on an itinerary. Ferries from Kasaoka cross to a small Seto Inland Sea island with beaches, granite hills, fishing-village lanes, and views across the Kasaoka island chain.
The island is especially good in warm weather, when swimming, kayaking, and simple beach time make sense. Even outside summer, it works as a slow walking day if you want the Inland Sea without the crowds of better-known islands.