Grönsö is a privately owned 17th-century manor on an island-like tongue of land in Lake Mälaren. The draw is the whole setting: old interiors, farm buildings, orchard, park, and lake water around the estate.
You normally see the house by guided tour, while the garden and grounds are easier to fit into a loose day. It pairs well with other countryside stops south of Enköping, especially if you have your own wheels.