Start at Sainte-Chapelle, ideally with a timed ticket for around 2:00–3:00 p.m.; allow about an hour. Its jewel-like upper chapel is best when the late-afternoon light catches the stained glass. Admission is usually about €13, and security screening can add 15–30 minutes, so arrive early. From there, walk a few minutes through Île de la Cité to Notre-Dame Cathedral. Entry to the cathedral is free, though reservation or timed-entry arrangements may apply; allow roughly an hour, and remember that large bags are not practical. Take a little time outside to appreciate the restored façade and the quieter riverside paths around the island.
Cross toward Saint-Germain-des-Prés—about 20 minutes on foot, or take Metro line 4 from Cité to Saint-Germain-des-Prés if you prefer to save your legs. Have an early dinner at Le Procope, Paris’s historic literary café-restaurant, around 6:00 p.m.; reserve ahead, and expect approximately €35–€60 per person. Afterward, stroll along Rue de Buci and Boulevard Saint-Germain to Café de Flore for a classic coffee stop—budget €10–€20, with the terrace ideal for people-watching. Finish with a one-hour Seine River Cruise, checking the evening departure point and timetable in advance at Pont Neuf or near the Musée d’Orsay. Evening sailings commonly run into the night and cost roughly €15–€25; arrive 20 minutes early, and bring a light layer, as the river can feel cool even in September.