As a sidetrip, take the D909 east to the Mont Veyrier cable car, continuing on to the pretty town of Menthon-Saint-Bernard and its medieval castle high above the lake.
The sidetrip involving a cable car is speedy and efficient. 
mnli hypothesis: The sidetrip involving a cable car is speedy and efficient.  premise: As a sidetrip, take the D909 east to the Mont Veyrier cable car, continuing on to the pretty town of Menthon-Saint-Bernard and its medieval castle high above the lake.