Here's a great 2-for-1 deal from San Francisco to Europe. Fly from San Francisco to London LHR for $270 roundtrip on Icelandair, with a full 24-hours to explore Iceland on the way back. This price requires the overnight in Reykjavik. Slightly higher $343 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 San Francisco SFO on Monday, October 22, returning from London LHR on Thursday, November 1. Make sure to choose a 1-night stopover in Iceland on the return flight, which will arrive back in San Francisco on Friday, November 2, after an overnight stay in Reykjavik.
Other dates are available for travel from October through mid-December. Travel to London must be on Mondays and travel from London must be on Thursdays for this price. A slightly lower $263 roundtrip fare from San Francisco 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 booking on Icelandair.com, you can get a 20% discount on checked baggage fees during or after booking. Online checked baggage fees start at $76 each way. Higher fees apply at the airport.
You can also find this price on Priceline, but you'll be subject to higher checked bag fees at the airport if you need to check bags.
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.