Loyalty Lobby reports a nice airfare deal on Delta/KLM/Air France of just $398 round trip between Boston and Valencia, Spain. That’s a great fare to fly into Valencia. More details including fare construction, rules, and example dates can be found at Loyalty Lobby.
While looking at that deal, I stumbled on a few other great airfares between Boston and Spain (and also to Paris). Skyteam has flights to Madrid and Alicante from $398 in September and October. See Google Flights for example dates:
However, if you’re willing to accept an extra connection each way, you could fly TAP Portugal for $379 round trip (click the image to go to Google flights):
But there are some real steals if you’re willing to book separate tickets and fly out on TAP Portugal and back on Norwegian. For example, you can fly round out on TAP and back on Norwegian from $304 total between Boston and Madrid at the beginning of November, or spend ~$350 for mid-October:
Early November weather in Madrid can easily be in the low 60’s during the day — definitely not summer weather, but decent for the price.
TAP has a number of other cities on sale as well. For example, you could also fly Boston to Paris on TAP Portugal both ways from $350 towards the end of October:
Loyalty Lobby posted a great sale to Valencia, but there are some other particularly good fares as you reach the end of shoulder season. A lot of other cities are available under $400 — play with Google Flights for more options.