An Island in the Oslofjord. There used to be a camping site there (The Rough Guide 2002 gives a phone number but it no longer exists) but now you just catch the ferry out and do your own thing. It's safe as houses. Frequented by overnight school expeditions and work parties, it's still big enough to avoid them all and enjoy great views of Oslo and the surrounding hills. Plenty of tracks for walks as well. A nice break from the city which all feel the same after a while. Bring your own food and drink.
Catch the Vippitengen bus outside the train station. This brings you to the docks where you get the ferry. You can get a special city travel pass that covers all of this. It's fairly cheap for Norway.
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