While the vineyards of Bordeaux and Tuscany rightfully command the spotlight for much of the year, there is a dramatic, terraced river valley in Portugal that stages Europe’s most captivating autumn spectacle. September and October mark the arrival of the Vindimas—the traditional grape harvest—and nowhere is this energy more palpable or visually stunning than in the Douro Valley.

As the days begin to grow shorter and the crispness returns to the air, North America transforms into a spectacular tapestry of color...