Icelandair has put Boston to Europe on sale for several cities during January to March 2017. Probably the best deal is $388 round trip to London LGW. Fares range from $348 to Copenhagen, Denmark; $356 to Oslo, Norway and $392 to Stockholm, Sweden for nearly every date January 5 to March 31. Paris for $487 is also a great price, but Paris seems to be on sale frequently these days.
Icelandair Boston BOS – Reykjavik, Iceland KEF – London Gatwick LGW
Icelandair operates like a legacy carrier with free carry-on, free checked bag and no charge for seat assignment. On top of that is the option to book a near free stopover in Iceland when booking your ticket. Winter tends to be a better time for seeing the aurora borealis-northern lights in Iceland.
Boston to London Iceland Stopover Ticket
Icelandair Boston BOS – Reykjavik, Iceland KEF (4 night stopover) – London Gatwick LGW (7 nights) – KEF – Boston BOS
$404.27 round trip Mon Feb 6 – Sun Feb 19
Ticket price increases $16 from $388 to $404 when you add a stopover in Iceland. My searches for stopover tickets this past year show a fare increase of $5 to $20 when an Iceland stopover is added to the ticket.
Keflavik Airport, Iceland (July 2015). I flew WOW from Boston, unfortunately before they also allowed free stopovers in Iceland.