Oscarsborg Fortress is a place set on two small islands nearby Drøbak. There is the fortress museum that introduces visitors to the history of thus place that has subterranean tunnels below the fortress, obstacle courses, bathing beaches and crab fishing.
There is also theatre, opera, concerts, a spa, an art gallery and many places to eat.
There is a number of activities for children including a children’s history safari through the tunnels, and a playground.
You can reach the island by ferry from Drøbak or Oslo, or by your own boat to Oscarsborg Marina.