Oslo Summerpark turns the forest above Holmenkollen into a warm-weather ropes and zip-line course. The routes run through the trees with a mix of bridges, balance elements, and aerial crossings, so it is a good family stop when the ski jump and museum are not enough activity for the day.
It is seasonal and weather-dependent, so check opening times before heading uphill. Combine it with Holmenkollen or the Nordmarka trails rather than treating it as a standalone city-centre attraction.