The location of Storholmen is central if you look at the map, but due to that there aren’t any bridges that lead there, it hasn’t been discovered by the masses for the gem that it is. The island was bought by a friend of Ivar Krueger (as in the Krueger crash) called Gunnar Kassman in 1917. He built a magnificent castle surrounded by a beautiful park and during WW2 it was the residence of the Soviet ambassador. Since then, the castle – and even the park – has degenerated.
The island has more history to offer than that: an old pine stump is protected by an elegant copper tent. In the 18th century, Count Lewenhaupt hid from the infuriated king under this pine. King Fredrik I was angry at the Count for handling the war with Russia in a poor way, but the Count was found and beheaded just a few days later. Apart from walking in the historical surroundings and enjoying the nature of the most inner part of the archipelago, Storholmen is nice for swimming if the water temperature allows it.
Take the subway (red line) heading north from the Central station to the final stop Ropsten. From there you can get to Storholmen by a small ferry boat. It is wise to check the times of the ferries with the operator in both directions as there are just a few departures each day (spring, summer and autumn).