Sweden is a beautiful country with vast forests and many islands. In the summer, it is a cooler destination with lots of urban and wild exploration. During the winter, Sweden becomes a wonderland covered in snow and ice. Sweden is one of eight countries in the world that have land lying within the Arctic Circle. Because of this, when visiting, it is possible that you may have an extremely long day or what feels like an endless night. The farther North you go (in any country) the more extreme daylight changes are. As far North as the Arctic Circle, summer will have almost zero darkness and winter will have almost no daylight. While that may be a hard thing for many people, unless you are planning to go into the wilderness of the Arctic North you probably won't experience this.
Southern and Central Sweden have some amazing cities to visit. The capital city of Stockholm has gained more notoriety in recent years and is quickly becoming a popular destination. The city offers many sights to see, food to eat, and a great nightlife scene. If you're in Copenhagen, you may want to hop on the train over to Malmö. It less than an hour to the Swedish city from Copenhagen and it has lots to see. The architecture is amazing with iconic buildings such as the Turning Torso. I found myself in Malmö this past summer and had a great time there. The city has paths to easily bike or walk and there are waterways both natural and manmade running throughout the city.