Skip the Blue Lagoon.
We did an off road tour day (went in Jan. or Feb.--I can't recall) and it was pretty incredible, but very intense. We saw many people who were out of their depth and got very, very stuck. So if that's your thing, consider going with a professional.
If you want to see the Northern Lights, go earlier. Also, book that for your first day. Most of the tours came with a guarantee (though maybe that's only because we were in prime aurora season) that if you didn't see the Lights, they would send you out a second day to try again for free, so you want to make sure you have enough time to rebook. We did both a bus tour (barely saw any lights) and a boat tour (amazing, right up until the camera went overboard into the Atlantic). Boat tour is the coldest I've ever been, despite pretty good gear, but incredible.