Boston Harbor is sprinkled with 34 islands, many of which are open for trail walking, bird-watching, fishing, swimming and camping. You can't visit them all in one day, so consider the activities on offer and plan accordingly.
Hop on the first ferry to Georges Island, where you can spend the morning exploring Fort Warren. After lunch, take the shuttle to one of the other islands. Hit Spectacle Island for good walking trails and marvelous city views. Head to Lovells to catch some rays on the otherwise empty beach and cool off in the refreshing Atlantic waters. Or venture to Grape Island to hunt for wild berries.
Catch a shuttle (if necessary) and ferry back to the mainland from Spectacle or Georges Islands.