These are the two islands the rest of Paris grew out of, and you can walk both before lunch. Under three kilometres: the Cité for the monuments, Île Saint-Louis for the quiet, and the river the whole way round.
Start with Sainte-Chapelle, early enough to beat the gate queue — the upper chapel's wall of thirteenth-century glass is worth the alarm. Out and east to the Notre-Dame square, whole again since the December 2024 reopening; read the façade from the parvis even if you skip the door. Then double back to the island's western tip and drop down into the Square du Vert-Galant, a pointed park at water level where the river forks around your feet.
Cross the Pont Neuf — the oldest bridge in the city, against its own name — and take the quays east to the second island. Rue Saint-Louis-en-l'Île is its single spine, slower and lower-key than everything behind you. End with Berthillon, made on the island; if the shop is shut for August or a Monday, a nearby café scoops the same tubs. Pont Marie takes you out.

