Via SecretFlying you can book New York or Boston to several cities in Europe and return from Lisbon for as low as $308 roundtrip.

The fare appears to be available for September through November travel.

• Book from Boston or New York to Berlin, Venice, Paris, Madrid or Barcelona
  
• Travel is on Star Alliance member TAP Air Portugal
  
• Use multi-city search. Return from Lisbon to either Montreal or Toronto
  
• You can route back to Canada via New York or Boston and get off the plane there, throwing away the Canada segment.

Las Ramblas in Barcelona


Roses, Spain about Two Hours Outside Barcelona

Here’s an example, New York to Venice and back to New York from Lisbon:

This will be bookable through most online systems, such as Orbitz, since it comes up using ITA Software searches.

It’s even possible to check bags, since when you arrive back in New York or Boston you have to claim your bags and walk them through customs before dropping your bags back off for the flight to Canada. Instead of dropping off your bags, you just leave the airport in New York or Boston. That’s called ‘Throwaway Ticketing’ and the New York Times Ethicist column says it’s ethical.

Here’s another example, Boston to Madrid and back to Boston from Lisbon:

Taking a look at the fare breakdown, it appears what is happening here is that flying TAP Air Portugal back to Canada drops the fuel surcharge out of the fare. (See: Explaining the Mystery Surrounding Fuel Dumps)

You want to make sure, on the return, that you book a connection back through your original departure city!

In any case don’t buy other non-refundable travel based on these tickets for a few days to make sure TAP Air Portugal honors the sale.