Hi! Here are some thoughts.
– March is a month where the hours of daylight change quickly! See the data here. Or see our post about hours of daylight in Iceland.
On March 1st, you’ll have good darkness from about 9:30 PM until 6 AM. On March 31, it’s only from 11:30 PM to 3:30 AM. If you want to see the northern lights, check the forecast for aurora activity.
– Whale sightings are less common in the winter. But check the Diary of a company in Reykavik when it gets closer to your trip to see if they are having any luck. (And check out our Whale Watching tips too!)
Okay, on to an itinerary. Are you comfortable driving? The Golden Circle and South Coast should be accessible, barring a winter storm. Take a look at our 1 week tour of Iceland with kids. Skip the Westman Islands, keep a close eye on road conditions at road.is and safetravel.is, and be ready to change your plans if they roads don’t look good.
You can see the road conditions from March 2, 2016 here. There are spots of ice in South Iceland, but everything looks easily passable if you’re comfortable driving in winter conditions.
Let me know what you think, and any questions this brought up!