Three days is enough to cover Paris's essential landmarks, its two greatest museums, and a proper feel for how different the city's neighborhoods are from each other. This itinerary front-loads the icons, gives day two to art and the Left Bank, and saves day three for a slower, local pace plus an optional day trip.
Start early at the Eiffel Tower before the queues build — book a timed entry in advance if you want to go up. Walk along the Seine toward the Arc de Triomphe, then head down the Avenue des Champs-Élysées. In the afternoon, cross to Notre-Dame Cathedral on Île de la Cité, then wander into the Marais for dinner — it's dense with small bistros and doesn't feel like a tourist strip.
Give the morning to the Musée du Louvre — pick two or three wings rather than trying to see everything. Cross the river for lunch in Saint-Germain-des-Prés, then spend the afternoon at the Musée d'Orsay for its Impressionist collection, followed by a walk through the Luxembourg Gardens. In the evening, take a Seine River Cruise to see the illuminated landmarks from the water.
Spend the morning climbing through Montmartre to the Basilica of Sacré-Cœur for the best skyline view in the city, then get lost in the surrounding cobbled streets. If you have energy left, the Centre Pompidou or Sainte-Chapelle's stained glass are both worth an hour. Travelers staying a fourth day should use it for the Palace of Versailles, a short train ride out.