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You don't have to bring a lot of warm clothing, but you have to pack sensibly. For example, a wick-dry inner, light natural fibre sweater, a down layer, and a windproof outer will do nicely. Hat, gloves, warm socks, and windproof pants (with long thermal underwear) would also be needed for that weather. A scarf is helpful to keep your neck warm and to cover your nose and mouth if needed, especially if it's windy.
Your usual ATM card should work fine in Sweden, there's probably no need to carry much US cash with you. Check with your bank first (tell them you are travelling!), and make sure you have a 4-digit PIN (at one point I had a 6-digit PIN and it was incompatible with a lot of systems around the world).
It's a good idea to book at least your first night in your initial destination. You can work things out from there (eg. Stockholm should be no problem). If you're going to northern Sweden, there will be far fewer services available due to the lack of population, and you'll definitely want to book ahead. Couchsurfing in a remote area is not terribly reliable.
These days it's super useful to bring a mobile device with WiFi networking (even if it's a phone and the phone part won't work), as you can find WiFi available in lots of places in the city. And if you have WiFi, you can use Skype (with credit) to call normal phones in Sweden.
Travel insurance (eg. in case you get hurt) is strongly recommended.
Presumably your passport has at least three months remaining validity (if you're a US citizen you won't need to pre-apply for a visa, but for other passports you might). The absolute minimum you need is your passport and your wallet.
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Regarding ATM cards, you should have one with chip, since most ATMs here only accepts those cards.
Upvote:5
Adding options about how to travel in Sweden. (I guess you already went....) To see the aurora Abisko is probably the best place to be.There are two main ways of getting up there.
Photo by flickr user beefortytwo.
A good site for planning your domestic travels in Sweden is resrobot.
More detailed information also available in this question about travelling from Berlin to Abisko.