Paris in July is a slog and Paris in December is a photograph; Paris in the second half of September and the first fortnight of March is neither, and it is when the city is most honestly itself. The rentrée — the return from the August holiday — is complete by mid-September, which means the small restaurants have reopened and the neighbourhood rhythm has resumed, but the tour buses have not yet begun the Christmas market circuit. Rooms at the good hotels drop by twenty to thirty percent from their October peak. The weather holds through the third week of September in most years.
The temptation on a first return trip is to base in the 1st or 6th arrondissement, and the temptation is worth resisting. The 6th around Saint-Germain is now essentially a luxury retail street with restaurants attached; the 1st is functional for the Louvre and little else. The 7th, quieter and more residential, remains a good compromise for first-time visitors — the Rue Cler market street on a Saturday morning is genuine, the walk to the Musée d'Orsay is ten minutes, and the rooftops facing north-east catch the light on the Invalides.
For a second or third visit, the 3rd (the upper Marais above Rue de Bretagne), the 9th (around Rue des Martyrs and the base of Montmartre), and the 11th (around Rue Oberkampf and the Marché Bastille on Sundays) are the quartiers where the city still lives at its own pace. A stay in the 3rd puts one within walking distance of the Picasso Museum, the Musée Carnavalet after its refurbishment, and the Marché des Enfants Rouges — the oldest covered market in the city, best on a Sunday around noon.
On hotels, Le Bristol on Rue du Faubourg Saint-Honoré remains the honest first choice for a serious stay, and the room selection matters more than it might elsewhere. The Deluxe rooms facing the courtyard are quiet but small; the Prestige Suites on the courtyard side deliver space without the noise of the street; the Panoramic Terrace Suites on the sixth and seventh floors, expensive as they are, have private terraces facing Sacré-Coeur that no other hotel in the 8th can offer. The pool on the sixth floor, tiled in teak and designed to look like a yacht deck, is closed to non-residents and worth an hour on a rainy afternoon. Epicure, the ground-floor restaurant, holds three Michelin stars and books six weeks ahead; the courtyard lunch at 114 Faubourg is the easier reservation and, in September, one of the finest lunches in the city.
A counterpoint: Claridge's is not in Paris, but a return leg through London on the Eurostar makes a five-night trip that splits three and two, and the two cities are more different than they used to be. The Eurostar from Gare du Nord to St Pancras is two hours twenty minutes; the walk from Claridge's to Bond Street is five minutes. This is a specific kind of trip and not for everyone, but it is worth mentioning.
On the cafes with terraces, three that hold up: Café Charlot on Rue de Bretagne in the 3rd, which faces south and holds the sun until three; Café de la Nouvelle Mairie in the 5th behind the Panthéon, which is a wine bar rather than a cafe strictly speaking but does the job; and the terrace at the Musée Rodin's garden restaurant in the 7th, which requires a museum ticket but is one of the quieter places to sit in the entire city.
Book restaurants on the day for lunch and a week ahead for dinner. The neighbourhood places will find a table at two; the Michelin ones will not find one at nine.