When you visit Gripsholm´s castle. You have a very big parking space outside. Then you have to walk from the parking spot around 300 meters to enter the castle. But dont park your car at the bus parking spots, then you will get a penalty. I saw many cars which had done that and the drivers werent happy about it.
During the summer season you can go from Stockholm to Mariefred on a special day trip ticket. In 2006 this tickets cost 250 SEK and include train to Läggesta where you change to a old steam train for the last 15 minutes. In Mariefred you will then have some hours to walk around until you take a old steam boat back to Stockholm.
Tickets can be bought at Stockholm Tourist Centre, Sverigehuset at Hamngatan in Stockholm (phone +46 8 508 28 508)
The most romantic way to travel in Sweden :-) The s/s Mariefred is probably the ship that has trafficked the same stretch in Sweden longest, as she has done the Stockholm to Mariefred run since 1903! My father went on it as a child when this was the only way to get to Mariefred within a reasonable amount of time.
The Stockholm to Mariefred run takes a bit more than 3 hours but you can either have a coffee on board or dine in style in the little restaurant. It otherwise reaches Mariefred just in time for a late lunch. If you want to, you can return with her as well, but then of course you don't get to see much of Mariefred unless you spend the night and return next day instead.
You leave from Stockholm City Hall, and the journey passes several Mälaren islands and you are told when something interesting can be sighted, such as a palace. Then there are of course all summer cottage owners on various islands, traditionally waving something white as Mariefred comes chugging and whistles at them :-) I love steam ship culture...