Located in the Marais district, Saint-Gervais-Saint-Protais Church was completed in 1657. It features the oldest organ in Paris (1601) and Flemish-style wood paintings. The architecture is both Gothic and Classical, a very interesting mix.
Fun Fact: The famous organ inside has been played by 8 generations of the Couperin family, including François Couperin.
Address: 13 Rue des Barres, 4e, +33 1 48 87 32 02
Metro: Hôtel-de-Ville/Pont Marie