Långholmen is a tiny island in the southern part of central Stockholm which housed a feared prison from the 18th Century up until 1975. The island also housed a correction spinning house for women (including children) that were either homeless or criminals. Today visiting the island is a much more pleasant affair. There are several bathing spots on the island, my personal favourite being Långholmens strandbad, which offers exceptionally clean water, a shower, toilets and you can buy ice-cream in the vicinity.
Today the old prison houses a popular hotel (!). Friends of mine who have stayed there say that it is very nice, but the cells are quite small. One practical fact is that the hotel welcomes dogs and they will offer a blanket, bowl and treats for your dog. Regardless if you travel with a dog or not, the island and its beautiful neighbouring island Reimersholme are beautiful and tranquil spots for taking walks any time of the year.
One easy way to get to Långholmen is to take bus 54 from the central station in the direction of Reimersholme and get off at Högalidsgatan. The ride takes about 15 minutes and then it is a beautiful 10-minute walk to Långholmen.