On our last trip to Sweden we decided to do a layover in Iceland. Iceland air now offers for free, layovers in most European cities for days even, without it changing the price of your ticket.
My only complaint I have is that we didn't do a longer stay in Iceland. That will be on the agenda next time for sure.
We had a 5 PM flight from Denver and landed in Reykjavik at about 7 AM. (Which was midnight Denver time.)
When traveling with kids I highly recommend traveling at night time if possible (at least for long flights!) Even if they don't get amazing sleep, just a few hours to break up an 8 hour flight of having to keep them occupied is a big life saver.
After a couple of hours into the flight I told the kids as soon as it got dark it was time to sleep. And then Drew reminded me we were flying so far North that it wasn't going to get dark. That certainly made it a bit harder to get them to sleep, but they did dim the lights on the plane, and we made a little 'tent' for Tindra and got her some milk and told her it was bed time. It took longer than normal to get them asleep (naturally) but they did sleep for about 3 hours of the flight. Better than nothing! Some sleep was crucial since we were going to be arriving in the morning and had some exciting things planned that they would obviously need to be awake for!
Since we were only doing a one day layover in Iceland (and heading on to Stockholm in the evening) we either had time to head into Reyjkavik and explore there or go to the Blue Lagoon. I really wanted to go into the city, but I knew we were all going to be tired from traveling (and still not done traveling) so we decided the Blue Lagoon was the best option for us. And it did Not disappoint.
Right inside the airport (and its a small airport so super easy to find) theres little desks with information about booking a trip to the Blue Lagoon. Everyone spoke English and were really helpful. We booked our trip right there (bus fair to the Blue Lagoon and back to the airport is included in the price) and grabbed a snack, and 30 minutes later we were on a bus on our way.
The bus ride there was only about 25 minutes and it takes you right to the lagoon. Right as you get off the bus there is a building where you can store luggage/strollers if you are traveling, for a small price. All of our checked luggage went on to Stockholm so we didn't have to deal with that thankfully, but we still had several carry on bags plus a stroller and carseat that we checked. I had packed all of our swimsuits in one small bag so we brought that in with us and left the rest.