La Defense. We enjoyed our afternoon there way more than I expected too. Even in its generic skyscraper-laden modernity, there's something very French and flavorful about it. I could have spent two hours watching people sit and enjoy themselves on the steps of La Grande Arche.

Another vote for the Cluny museum. And especially relaxing and people watching in Luxembourg Gardens.

The Jardin Luxembourg as above. We visit every trip.

Ian

Parc de Belleville
Parc des Buttes Chaumont
La Promenade Planteé
Lady and the Unicorn in Musée Cluny
The Objets d'Art department at Musée d'Orsay
Musée Carnavalet
Musée Nissim de Camondo
Musée de la Vie Romantique
Parts of the Canal St Martin
Cours de Vincennes Market
Parc Montsouris and environs
Auteuil
Passy
Charonne
Cité des Fleurs across from Brochant Métro station
La Defense

During my recent trip to Paris, the Camondo museum and the Gustave Moreau were both new discoveries for me. The collection of 18th century objets, painting, furniture all beautifully display in the Camondo private hotel near parc Monceau, are some of the best example of France's most brillant period.
The Gustave Moreau, the first symbolist, created his own museum for his mansion, before his death. He collected thousands of his drawnings and paintings, the visit is definitely worth the time.

Concerts at Saint-Julien-le-Pauvre were also marvelous evenings.