Île de la Cité and Île St-Louis, 4th Arr.
Concierge.com's insider take:
Midstream in the Seine, the Île de la Cité is Paris's birthplace, where a Celtic tribe known as the Parisii built their wattle settlement around 250 BC. The island is bound to the mainland by four bridges, including the city's oldest—the now mislabeled Pont Neuf ("new bridge"). To get a sense of what the island was like before Baron Haussmann totally redeveloped it, visit the archeological crypt (1 Place du Parvis de Notre-Dame; 33-1-55-42-50-10), the pretty Place Dauphine (Between Pont Neuf and Rue de Harlay), and the side streets on the north side of Notre-Dame. Sainte-Chapelle, a masterpiece of Gothic architecture, is always mobbed for its stained glass; to beat the crowds, try going to an evening concert, especially in summer, when the days are long and the light through the stained glass is gorgeous until 10 p.m. (4 Blvd. du Palais). On the upstream tip of the island, behind Notre-Dame, the Deportation Memorial is a moving monument to French citizens who were deported and died in Nazi camps.
Developed in the 17th century as an exclusive enclave, half-mile-long Île St-Louis is lined by the mossy town houses of the old-money elite (the Rothschilds lord over the upstream eastern side). Plaques identify dozens of the artists, writers, and bigwig politicians who've lived here, from Charles Baudelaire to Georges Pompidou and Ernest Hemingway. Some of the richest, most irresistible ice cream anywhere comes from Berthillon, headquartered at 29–31 Rue St-Louis-en-l'Île (33-1-43-54-31-61; www.berthillon-glacier.fr), and is also sold by a half dozen island cafés and restaurants. And if you want to learn to fly-fish or spin-cast like a true Parisian, head to century-old Maison de la Mouche, on the Boulevard Henri IV, the roadway bridge that crosses the island's upstream end (1 Blvd. Henri IV; 33-1-43-54-60-46). Head to the island's south side for a great view of Notre-Dame's flying buttresses from the Quai d'Orléans.