Last weekend I took my family out to the archipelago outside of Oslo. The islands can be reached by ferry from City Hall Pier 4 close to Aker Brygge in summer. We took the boat for the cost of 132,- for the whole family. This will be included in your 24-hour ticket. I recommend you to start your day early, then you will be one of the few travellers and will have the islands for your self.
On random we hopped of on Gressholmen. Rambergøya and the northern parts of Gressholmen are nature reserves, and the bay between the two islands is an important nesting area for sea birds. From the late 19th century Heggholmen was a small industrial community, and Gressholmen was the location of Norway's first main airport established in 1927.
You can bring your picnic bag and sit on one of the bare rock and rest your eyes. You will see sailboats, cargos and fishermen. Don't be surprised if you find your self in a conversation with a family with small children canoing. Just remember its not allowed to tent outdoor in the archipelago, but thats not necessary - the boat leaves every 30. min and its just a 15 min ride back to Aker Brygge.
It was a really beautiful day in Oslo, so I meet my friend Kajsa for dinner at Cafè Skansen - so do as the Norwegians - when the sun shines, we really enjoy life.