Sweden Krona SEK
Norway Krone NOK
Denmark Krone DKK
They are not in Eurozon.
When we were in Norway they would take euros in the touristy places i.e. Flaam, Stavanger and Oslo but we asked first...otherwise use your credit card/debit card in the ATM and you will get local currency...it was very expensive in Norway so we didn't spend much!
Lots of retail outlets accept euros and some will give you change in euros.However,you never get a particularly good exchange rate and places like Post Offices (for stamps)very rarely accept euros. In my experience,it is best to have local currency for incidental expenses and use a credit card for larger purchases.As far as clothes go,take what you would wear for a British summer.
I did a baltic cruise last July and had DKK and SEK. i found that when I got in the taxi in Sweden, he said he took Euros, but when he told me the price to take me into stockholm city centre, it worked out a lot more expensive than the price he wanted if I was paying in kroner, so i was glad that I'd arranged small amounts of currency before I left (The Post Office is beat in my opnion). I used Credit card fro larger purchases. as for clothes take what you would for a holiday in Britain. Last year we had a bad summer here and so did they in the baltics, it was wet and windy and not very warm. However i was in denamark the previous summer and it was mainly dry and sunny but not that hot. certainly take a raincaoat and umbrell.