Single or multiple day trips from Reykjavik?

Sorry I can't answer your questions about the lagoon and ice caves, but would make just one point about the timing of your days 4 and 5. The Northern Lights tour goes out pretty late (of course, for darkness) and returns very late. The Golden Circle tours can leave pretty early (8:30 ish - you often have to travel from center of town to bus station to catch your tour). That makes for a short night of sleep (my motto is sleep when I'm dead, but that might not be yours). Also since you're at the whim of the solar flares, your northern lights tour could get rescheduled, so I'd book it early on in your trip in case you have to pivot.

FWIW I enjoyed my day trip to the South Coast with GeoIceland. Small group, great guide, small buss with WiFi (!!), and an efficient use of time.

Thanks for the answer! It makes sense to schedule the northern lights trip earlier just in case the weather is unfavorable. I too don't mind the lack of sleep while travelling, the fuller the day the better. Still I tried to alternate busier days with more laid back days. Hadn't heard of GeoIceland before I'll check their tours, thanks again!

Beginning of March the days are ten and a half hours long; by the end of March they are thirteen and a half hours long. You might think about going a little later in the month.

Your Day 4 is a little light. I don't know how much time you want to spend soaking in hot water, but I wouldn't think it would be all day. Perhaps days 3, 4, and 5 you could do the south coast tour, then do the Blue Lagoon and Reykjannes Peninsula on Day 6. If you have an early flight on Day 7, you might want to consider spending the night on Day 6 near the airport.

To me, the benefit of taking a multi-day tour instead of day trips is that on day trips you can only go so far and then you are doubling back to Reykjavik every day. With a multi-day tour you can go further and see more.

Thanks naalehuretiree! We want a winter trip with snow and all, we were in fact considering February but opted for March to get some more daylight. I don't really know when the snow starts to melt but I'd rather avoid the brown slush season and early March feels safer (hopefully). Plus the new moon will be in early March with higher chances of Aurora.
I agree that day some days are lighter and sparing more time for the south makes sense so the 3-day tip option is growing on me. Doing the lagoon after a long trip before going home would be pretty relaxing.

A few thoughts.

I would not stay in Reykjavik the entire time. Do a multi day tour up the south coast for 3 days. Those tours can include a glacier hike and ice cave as well as the golden circle. We have done both the glacier hike (February) and ice cave (October). The ice cave and glacier hike are two totally different experiences. As mentioned, the northern lights tours can go very late, so think ahead about whether you want to be up early in the morning for another tour. I would do the tour the morning after you arrive. If you are tired, you can sleep on the bus for a while. That way you would be back in Reykjavik and could schedule a northern lights tour for day 5. That would give you a buffer in case the tour doesn't go and you could go on day 6. That still is giving you two days in Reykjavik. One is enough, although we never tire of it. So you might consider a day tour to Snaefellsness for one of those days. While in Reykjavik, you might consider going to a local pool. We did Sundhollin in February during a storm and it was wonderful.

For the tours, I highly recommend Aurora Experts for the northern lights and Troll for the south coast. We had a 2 day tour booked with Troll and a big storm was rolling in, so the second day had to be cancelled. They were so good with communication and offering us options. We opted to go for one day and have the second day refunded. That refund was processed immediately. The way they handled that was so good that we went with them again for another excursion on another trip. For the NL's, Heimer is the owner/operator for AE and he was wonderful. He has a smaller Mercedes van so it only holds about a dozen people. He does not go to the usual spots where the big buses go, so you are not there with dozens of buses and hundreds of people. You are at a spot with just his group. He takes photos and sends them to you a few days after the trip, at no extra cost, and you can do whatever you want with them. He also is the only operator that has winter jumpsuits to keep you warm. He also has tripods for you to use, again at no extra cost. Other operators charge for this. He does the usual hot cocoa and traditional Icelandic snacks. We went with him this past October and if we opt for another NL trip, I would absolutely book with him again.

Thank you! You've pretty much answered everything I want to know. By now I'm completely sold on doing the 3-day tour, this many people advising to spare more time to the south can't be wrong. And who knows when we'd be able to visit again (although I'm already beginning to plan a summer vacation in the future), so it's best to make the best of the 5 days we'll be there. Your input about the ice caves only solidifies the decision.

I hadn't considered going on the tour on day 2 but it makes sense to have a more front-loaded program, with more relaxed days 5-6. I feel like going on another day trip in Snaefellsness would overcrowd the program (with whale watching, spa, and exploring the city itself) but maybe we could decide then and there if we feel like we've seen enough of Reykjavik and run out of things to do.

It's good to hear that you recommend Troll for the south coast, I have been making all plans and comparisons based on their itinerary, which seems to include pretty much everything I'd like to do. Their tour also happens to include a northern lights hunting night, but if we don't get lucky on that one I'll definitely consider booking with Aurora Experts.

Ice caves will be completely different experience from glacier hike. Glacier hike looks to be strenuous. We passed many hikers while walking to the glacier. We did the ice cave in Perlan which we thought was awesome and enough ice cave (it’s real, and freezing but indoors)! We toured with Your Day Tours and they were awesome. Do not miss the Blue Lagoon, or alternatively another thermal pool (people also liked Sky Lagoon) we felt rejuvenated and it was the highlight of our trip. You won’t miss anything by not sleeping in a quaint south coast village. Reynisfjara is absolutely breathtaking, otherworldly. We were there on a somewhat stormy day and most of the tours had just cleared out.

Sam--The glacier hikes are not strenuous. The photos can appear that way, but it is not. We have done both:)