Demonstrations in Paris rarely cause full-day site closures. When closures or disruptions occur, they are typically localized around major protest routes (like Place de la République, Bastille, Nation, or the Champs-Élysées/Opéra axis) or involve temporary Metro station bypasses.
But if you'd like some alternative places, voilà...
Musée Marmottan Monet (16th Arrondissement): Set in a former hunting lodge near the Bois de Boulogne, housing the world's largest collection of Claude Monet's works (including Impression, Sunrise).
Access: Metro Line 9 (La Muette).
Petit Palais (8th Arrondissement): Directly across the street from the Grand Palais. If the Grand Palais is closed or entry is delayed, step into the Petit Palais fine arts museum. The building features a stunning glass-domed garden cafe, and admission to the permanent collection is completely free.
Musée Jacquemart-André (8th Arrondissement): A 19th-century private mansion showcasing an exceptional collection of Italian Renaissance and French masters, preserved as a lived-in aristocratic residence.
Musée de l’Orangerie (1st Arrondissement): Located at the edge of the Tuileries Garden, famous for Monet's custom-built Water Lilies (Nymphéas) oval rooms.