Reggie (Audrey Hepburn) is a young widow chased across Paris by men who want her late husband's money, helped by a mysterious stranger (Cary Grant). Start at the Palais-Royal: its gardens and the Comédie-Française, the theatre where the finale takes place, are a three-minute walk apart. The American Express building, where two men fight on the rooftop next to the Opéra, is about 15 minutes north on foot.
On the Left Bank, Pont au Double is the Seine walk with Notre-Dame across the water. The building that plays the Hôtel Saint-Jacques, Reggie's hotel in the film, is on Rue Censier, about 15 minutes further south. To the west, the UNESCO headquarters, the agency where Reggie works as an interpreter, is about 10 minutes from Varenne station. Give it two days: the river and the Latin Quarter one day, the west the next.
At Varenne metro station, where Reggie escapes a man following her, look for the copy of Rodin's Thinker on the platform. Pont au Double also appears in Ratatouille.