Luggage and bags:
Since I lived there for 5 months, I brought 2 suitcases, my backpack, and my purse. But if you're not going to stay as long as I did, one suitcase and carry-on is good enough, unless you need room for winter sports equipment or anything like that.
Clothing/Shoes/Weather Gear: If you're going in winter, bring warm clothes. It gets very cold and snowy. Comfortable winter boots, a coat, gloves, and a hat. But bring trendy clothes for the clubs if you want to go out at night.
Toiletries and Medical Supplies: Your own prescriptions, and some over-the-counter medicines you might need. In Sweden, I think you need a prescription for most things (even things that we would normally buy over the counter), so bring your own tylenol and all that.
Photo Equipment: A normal camera and film. There are many photo stores in town where you can buy film or disposeable cameras, and get film developped.
Camping/Beach/Outdoor Gear: If it's winter, warm clothes! Snowpants and a ski jacket if you want to go skiing/snowboarding.
Miscellaneous: If you wear glasses or contacts, bring a spare pair, in case you lose or break your normal pair.
There is no special rainy season
Avg. Temp. in Spring: max.: 2 – 13°C ( 34 - 55°F ); min: -3 - 3°C ( 27 - 38°F )
Avg. Temp. in Summer: max.: 17 – 20°C ( 61 - 68°F ); min: 8 - 11°C ( 47 - 51°F )
Avg. Temp. in Autumn: max.: 1 – 12°C ( 34 - 53°F); min: -3 - 5°C ( 27 – 41°F )
Avg. Temp. in Winter: max.: -4 – -2°C ( 25 - 28°F); min: -11 - -9°C ( 12 - 16°F )