Last year we traveled to Paris on IcelandAir. We took advantage of the extended layover option and spent 2 days enjoying Iceland on our way to Paris. There were no additional fees on our flight…and the round trip was also very reasonable! Here’s more on what IcelandAir offers: https://www.icelandair.com/flights/stopover/?gclid=EAIaIQobChMI9YqnwoTw2gIVRgOGCh0-dAkIEAAYASABEgL7sfD_BwE
Here are some more resources on extended layovers:
https://thriftynomads.com/airlines-offer-free-stopovers/
http://www.travelandleisure.com/airlines-airports/airlines-with-free-stopovers
https://www.savoredjourneys.com/how-to-book-a-free-stopover/