Unlike Hovedøya's manicured lawns or Lindøya's cabin villages, Gressholmen feels wilder with forest trails, sea cliffs, and fascinating aviation history. From 1927-1939, this was Oslo's main seaplane airport before Fornebu and Gardermoen existed. You can still see the original hangar and slipway where planes were hauled out of the water. Today, the old airport area is a flat grassy plain perfect for picnics and ball games, while the rest is hilly and forested.
Large parts of the area are protected as a nature reserve while a marina, a handful of houses and a seasonal island restaurant occupy the more developed shorelines. Heggholmen Lighthouse sits at the western island's tip—one of the inner fjord's oldest lighthouses. The incredibly photogenic wooden building with red top perches on a rocky outcrop. The view from here back toward the city skyline and Opera House is fantastic.
Gressholmen Kro operates in a charming yellow wooden house near the ferry dock, a historic summer restaurant with large outdoor seating serving classic Norwegian summer food like mussels, shrimp sandwiches, and cold beer. Often less crowded than Hovedøya cafés but weekends create 20+ minute drink waits and 50+ minute food waits as it becomes a "tourist trap" on sunny days.