Here's a great 2-for-1 deal from Seattle to Europe. Fly from Seattle to London LHR for $286 roundtrip on Icelandair, with 24-72 hours to explore Iceland on the way back. This price requires the overnight in Reykjavik. Slightly higher $347 roundtrip fares are available if you don't want the overnight layover.
For example, I was able to find seats using Icelandair's stopover search departing Seattle SEA on Tuesday, February 5, returning from London LHR on Wednesday, February 13. Make sure to choose a 2-night stopover in Iceland on the return flight, which will arrive back in San Francisco on Friday, February 15, after a 2-night stay in Reykjavik for Valentine's Day.
Other dates are available for travel from mid-January through March. For this price, travel to London must be on Sundays, Mondays, or Tuesdays, and travel from London must be on Wednesdays, Thursdays, and Fridays. A slightly lower $279 roundtrip fare from Seattle to London-Gatwick LGW is available with the same travel rules requiring the overnight layover in Reykjavik.
This is Icelandair's Economy Light fare, which only includes a standard sized carry-on bag plus one personal item. If you need to check bags, pay $100 more for the Economy Standard fare, which also includes stopovers up to 7 nights.
You can also find this price on other third party sites by using a multi-city search tool to build in a stopover in Iceland.
Need a room in London? Here's what's available for the above dates on Hotelwatchdog. For your stopover in Reykjavik, have a look at Hotel Jazz, which is close to the airport and the Blue Lagoon hot springs.