If you were just walking by Clos Montmartre on a trip to the Sacre-Couer, you might assume it was just a particularly lovely community garden dotted with peach trees and vines. Actually, the Clos is the oldest working vineyard in Paris, and on clear days, from here you can see all the way out to the Eiffel Tower.
The best time to visit Clos Montmartre is during Fête des Vendanges — the harvest festival — when the grapes from the Clos are taken over to Montmartre town hall to be fermented and turned into around 1,500 bottles of gamay and pinot noir.
At rue des Saules and rue Saint Vincent, you'll need to arrange a visit to Clos Montmartre with the Montmartre tourist office, which is close by at Place du Tertre. The tour is free, though you pay for the tasting, and generally it's better to be in a large group if you want to gain entry. For the harvest festival you can stroll right into the working vineyard. It's generally held in early October, but do check in advance.