I second the Galleries Lafayette and Marais suggestions.
You can also "shop" for outfits by watching the stylish French women around you. It's convicting and inspiring at the same time.
And to save posting time...
A red bus tour will help you to get the lay of the land in a few hours
Enjoy a chocolat chaud. Yum!
Macaroons at Laduree melt in your mouth, should you choose to indulge.
Have tea at the Ritz, if you have time. Catherine's isn't too far away, and they have tons of perfumes and will hook you up with the perfect fragrance for you.
L'Atlas Moroccain is a great restaurant in the 5th, if you like Moroccan food.
Visit Notre Dame and Sacre Coeur.
Will you have time to get out of the city? If so, then take the train to Versailles.
You will find many other tips on Fodors.com under the European travel message boards...a wealth of information from other travelers.
Bon Voyage!