Without hesitation, I must say The Blue Lagoon. I’m sure many people will back me up, but it’s such an industrial location that it’s quite mind blowing.
You’ll find that your skin feels fantastic for some time after too.

I think you’ll find it difficult to see the Northern Lights at that time of year or that would have been my second suggestion (though it snowed when I went and I have yet to see them!) with the constant day light, but I guarantee, you won’t be disappointed if you head for the Lagoon.

Hope that helps.