I really enjoyed Icelandair and I wouldn't hesitate to fly them again. Reykjavik is an interesting city with engaging, very intellectually gifted people living there. I would urge you to take Icelandair up on the stopover idea. The days are quite short in late December, it's true. But you may luck out and get to see the most stunning display of Northern Lights possible. Also, the weather is actually WARMER in mid-winter, on the average, than Minnesota. Typical daytime highs are in the low to mid-30s F at the coldest time of the season and dip only to the mid to high 20s at night. Give it a go!