We have a trip planned to Iceland about the same time. Coming from the subtropics neither of us are willing, or confident enough, to drive in the winter weather, and so many places we want to see are a long drive from Reykjavik where we'll be based, so we're taking several small-group tours, most likely to Golden Circle, South Coast (Jokulsarlon Glacier Lagoon), Snaefellsnes Peninsula. We would rather let an experienced winter-weather driver(s) be at the wheel of those 12-14-hour trips, a lot of which will be in the dark. We'll be doing a few Northern Lights tours (as no guarantees with the NL that we'll see them first go), again small group ones, then we can rely on someone else to do the monitoring and knowing where to go (again winter-weather driving in the dark!).
In Reykjavik, we looking at a visit to Perlan (hope the planetarium will be open by then), a chocolate factory tour (it says "endless samples" on the website - we'll see!
), a whale watching tour, and also the free walking tour. Can't say any of them are "must-see"s though. These things we'll be doing during the 'recovery' time from the longer tours we've allowed ourselves as we're older and need to rest our bones. I think this might be an idea (recovery time) if you are the ones doing the driving, especially after the longer drives.