Mariefred is a small town named after the Carthusian monastery Pax Mariae (latin for Mary's peace - 'fred' means peace in Swedish). The monastery is long gone, demolished in the 1500s after the Protestant Reformation, its bricks reused for building the Gripsholm Castle.
Drive southwest from Stockholm along the E4/E20 to Södertälje, continue along the E20 in the direction of Göteborg. About 40 minutes from Stockholm.
Or take the train to Läggesta, and bus - or museum railway - into Mariefred.
Or, in summer, travel by steamship Mariefred which starts near the Stockholm City Hall in the morning, takes 3-4 hours to Mariefred, and returns to Stockholm in the evening. S/s Mariefred has travelled the same route for more than a century!
You'll walk through the entire town in about 7 minutes.
Gripsholms slott - Gripsholm Castle. The oldest remaining parts are from the early 1500s. Amazing interiors, including an 18th century theater. Houses the Swedish state's portrait collection. And don't miss the stuffed lion!
Grafikens hus - international center for fine arts printmaking.
Narrow gauge railway museum.
Very quaint wooden houses and town square, lake views.
Kärnbo, 12th century church ruin with a nice view of the town (right behind Grafikens hus).