Paris is shaped by the Seine and its arrondissements. A day around the Louvre is nothing like a day in the Marais, on the Left Bank or in Montmartre. Book near the streets and the Metro or RER station you will actually use.
Around the Louvre and Tuileries, you can walk to the Seine, Palais Royal and Musée d’Orsay. Le Marais has Place des Vosges, small streets and late cafés. The Latin Quarter sits by the Panthéon and Jardin du Luxembourg, and Saint-Germain-des-Prés has the Left Bank galleries and bookshops.
Montmartre rises above the city around Sacré-Cœur and Place du Tertre. Its streets are steep, and the main approaches fill with visitors. Canal Saint-Martin is farther from the big monuments and better for a slower neighborhood stay. In the 7th arrondissement the Eiffel Tower and Invalides are close, but getting across to the Marais takes longer.
The city has several large rail terminals. Gare du Nord is the Eurostar arrival point and has RER B for Charles de Gaulle Airport. Orly is on Metro line 14. A central hotel helps, but one near the right station can spare you a cross-city transfer with bags.
You can walk between neighboring quarters and take the Metro or RER for anything farther. In May, the average high is about 70°F. July and August are closer to 80°F. Rain is possible all year.