Sherkin Island lies just across the harbour from Baltimore, a ten-minute ferry ride away. It's about five kilometres long and two and a half wide, with around a hundred residents, one pub, a church and a community centre. Its sights are a ruined abbey and some fine sandy beaches.
Ferries run hourly from Baltimore year-round, and the island is small enough to walk across, so you don't need a bike or a car. It makes an easy, relaxed half-day.