In half a century of traveling every corner of the world, I cannot think of a city on earth that tops Paris. It's got some problems, but the architecture, street food, gardens, etc beats all. A central dropoff is plausible since there is vast room for busses in Place Concorde. Near stunning gardens as a place to spend leftover minutes before pickup. Another cruise line describes this tour thusly:
"Enjoy a half day of exploring Paris all on your own. Your tour begins with a scenic drive to the city center. You’ll disembark at Cours-la-Reine Park and promenade near the Place de la Concorde, a favorite place for shopping and sightseeing. City maps will be provided to help you plan out your day. Feel free to explore the Arc de Triomphe, the world’s largest triumphal arch, and get a glimpse of the famous Eiffel Tower. See the Notre Dame Cathedral from the outside and admire its beautiful Gothic architecture. If you’re an art lover, the Louvre Museum is a must, as it is the home of Venus de Milo and the Mona Lisa. After your day of exploring the City of Light, you’ll rejoin your motorcoach at Cours-la-Reine at the agreed upon time for a relaxing ride back to the pier."
Even a remote dropoff may give a charming walk to center, where downsized attractions can be knocked off like intensely flavored morsals: dazzling Sainte Chapelle, Monet's water lily museum, snacks and sublime ice cream on Rue Sainte Andre des Artes, etc.