This park is in a quieter part of town, and mostly enjoyed by locals. The name Idioten, idiot in English, comes from the fact that there used to be a home for mentally disabled kids across from the park. Not very politically correct, but the historical nickname stuck, and Idioten it is.
Idioten is located in the Adamstuen neighborhood, with beautiful old houses and apartment buildings. A wander is recommended; it is not a tourist hotspot so it gives you a good feel of how people live (and lived) in the Western part of Oslo.
The park itself is located on a slope. This makes it hard to gather in big groups, but perfect for lounging on the grass in the sun. If you go to the top, you can sit down on a bench and see the Holmenkollen ski jump in the distance.
If you fancy a coffee or a snack, stop by Åpent Bakeri Adamstuen, which is located on a small square across the road from the entrance to the park.