Concierge.com's insider take:

Just a seven-mile ferryboat ride across the Boston Harbor, this little-known national park makes a great day trip on a sunny summer day. The park extends to 34 islands, but only six are accessible to visitors. Fort Warren on Georges Island was built in 1833, and served as a military training ground and a Civil War prison. Nature lovers will enjoy Grape Island, which has a multitude of shorebirds and berry bushes. Ferries depart from Long Wharf in front of Christopher Columbus Park and run between Georges, Lovells, and Spectacle islands; interisland ferries shuttle to Grape, Bumpkin, and Peddocks islands. The schedules vary depending on the season and the day of the week; be sure to check the schedule online in advance). And pack a picnic lunch, as food options on the islands are slim (just hot dogs, subs, and sodas on Spectacle and Georges).

Early May–early October only.