A 12-hour layover at Paris-Charles de Gaulle (CDG) or Orly (ORY) is the perfect amount of time to venture into central Paris. After clearing customs and storing your luggage at the airport's "Baggage du Monde," you can take the RER B train, which reaches the city center in about 40 minutes. You could start with a visit to the Île de la Cité to see Notre-Dame (viewable from the outside as restoration continues) and the nearby Sainte-Chapelle. From there, a 1-hour Seine River cruise provides a quick way to see the Louvre, the Eiffel Tower, and Musée d'Orsay from the water. You would have plenty of time for a classic French lunch at a bistro in Saint-Germain-des-Prés or a walk up to Montmartre for panoramic views of the city from the Sacré-Cœur. Just ensure you head back to the airport at least 3.5 hours before your departure to allow for security and potential traffic or rail delays, as CDG is famously large and busy.