Paris - Shopping

Le BHV Marais, a reasonably priced department store across the street from the Hotel de Ville. Nice cafeteria on the top floor.

Rue Saint Antoine from near Rue Castex to Rue de Rivoli near Rue de Pont Neuf is a very good not super luxurious shopping area.

The streets around the Centre Pompidou are loaded with friperies (thrift shops). Particularly Rue de la Verrerie. I got some good deals there last time I was in Paris.

or go to the famous Printemps and Lafayette, and now Samiritaine, or look for a passage for unusual shops

Look to bunch up purchases in a store--you can save 10-20% by doing VAT if you meet the euro purchase minimum. Get to airport a little early to process forms. Research how to do VAT. You may have to present your goods, so anything you purchase and want VAT back will be in carryon. However, if you look up VAT information for France, you can get the current rules.

If leaving from CDG-there is amazing duty free shopping. It all depends on what you are interested in buying. Frankly, I'd go to Galleries Lafayette or Printemps, or other stores posters here have mentioned, buy everything there and their well functioning in house VAT department is right there to do the paperwork you need for presentation at the airport. (Bring a photocopy of your passport when shopping, so they can fill out the passport number-- I never carry my passport around, just a photocopy and noone has given me a hassle).

If you check out Hermes you will be able to get an amazing scarf or twilly that will last a lifetime and there is nothing more Parisian than an Hermes scarf. Hermes is on St Honore and it's quite the place.
There is an Hermes shop at CDG and when I was there the prices were quite good--but the exchange rate was more favorable than now. But buying at the airport, everything is VAT free so you will save a few $.

Thanks a lot everyone for the comprehensive responses.