We just returned from a trip to Sweden. The weather was not all that good (you're in the north, of course) but we were lucky that it didn't rain too much.
First we visited the capital of Sweden, Stockholm. We stayed at a hotel where we had a wonderful sight from our room window, over all the canals. Stockholm is called 'Venice of the north' and it's true that there are a lot of waterways and small islands.
Next we took a ferry (it leaves at Nynäsham, some 40There minutes drive from Stockholm) to Gotland. Gotland is a bigger island in the Baltic Sea, at the height of Letland and Estland. We stayed there in the main town (well, town is a bit exaggerated) named Visby. Every 38th week of the year, this town celebrates the Medieval Week. Every inhabitant walks around in medieval dress and all the time there are tournaments and other historical events. This year they also did a reenactment of the Battle of Visby, where the Danish king Waldemar conquered the city.
Visby is UNESCO World Heritage, because it is a city that is completely encircled by city walls, just like it used to be in the Middle Ages.
The only remark I have about it is that they don't seem to like foreign tourists overall. An example: when you asked for a reservation in a local restaurant, they said it wasn't necessary. But when you arrived in the evening, you got to hear the restaurant was fully booked and there was no space!