Afternoon: Le Marais or Saint-Germain
Le Marais is the easier of the two to wander without a plan; Saint-Germain-des-Prés is the one to choose if you would rather sit and talk for two hours than keep moving.
Either fills an afternoon without a reservation, which matters more here than elsewhere: the evening starts so late that an over-planned afternoon just leaves you waiting.
Dusk: the river, or Canal Saint-Martin
A walk along the river is often the better half of the evening, and it costs nothing. Canal Saint-Martin is the quieter alternative and suits a slower pace; Montmartre is the one to save for daylight, since the climb is the point.
This is the hour that makes the day feel like Paris rather than like dinner with a view.
Dinner at nine
Meals are unhurried and nobody will move you along — a table is yours for the evening. Parisians dress simply but deliberately: understated rather than dressed up, well cut rather than formal.
If you would like the evening conducted mostly in French, or mostly in English, say so when you request availability. Paris companions speak English and several speak a third language.
Getting back, and when to come
The Métro runs until roughly 1am, later on Friday and Saturday, which fits a nine o'clock dinner without hurrying it.
Late spring and September into October are the strongest windows — the weather is kind and the city has not emptied. Many Parisians leave in August, which makes it quieter but closes a lot of neighbourhood places, so an August visit wants a plan rather than a wander.
Frequently asked questions
Do I need to speak French?
No. Paris companions speak English, and several speak a third language. Say which language you would prefer when you request availability.
What time is dinner?
Eight is early and nine is normal. Plan the afternoon lightly, because the evening starts later than most visitors expect.
Is August a bad time?
Quieter, but many neighbourhood places close. It works with a plan and disappoints without one.