Paris has more to see than any one trip can cover, so be selective. The major museums — the Louvre, Musée d'Orsay, Centre Pompidou, Musée Rodin — each deserve several hours. The classic monuments (Eiffel Tower, Arc de Triomphe, Notre-Dame) are essential but crowded; book tickets online in advance for the ones that require them.
The Paris Museum Pass (available for 2, 4, or 6 days) gives unlimited access to over 50 museums and monuments including the Louvre, Versailles, and most major sites, and allows you to skip ticket queues. It pays for itself quickly if you're visiting multiple major attractions.
Beyond the famous sights, the best of Paris often emerges from neighbourhood exploration. The Marais, Montmartre, and the Latin Quarter each repay an afternoon on foot. Père Lachaise cemetery is one of the most unusual and moving places in the city. The Buttes-Chaumont park in the 19th arrondissement is the great local secret, almost never crowded with tourists.