Kathy, I love Paris, and I've been lucky enough to go three times in the last four years! I never had a lot of money to splash out on designer stuff. I'm a fan of the Left Bank, and the things that I think the French do really well are accessories. Of course there are hand bags, but if you are looking for something cute and different, find a hat store, or a store devoted entirely to leggings and stockings, buy some gloves, or a cute pair of shoes. I also love fine paper stores, and there are a few around. In the Marais District, I can tell you my favorite shoe store: Hoses
http://www.hoses-limited.com/en/welcome.html
I have a belt that I bought in the south of France one year that still makes me smile. My current journal was purchased in France, too, and that makes it special.
I would suggest the non-material types of souvenirs, too, like really great gelato (Ilse St Louis), or a cheesy crepe (I like the ones by the Sorbonne, cuz the students know where to go where it's cheap and good!). Or a 5E drawing of an antique shoe from the vendors on the left bank of the Seine.
Get some ideas of what's fashionably cool out there, and you will be anticipating trends here a year in advance.
Gallerie Lafayette, Bon Marche, etc are very large department stores. They have great variety, and good eating, but do not offer the small store experience. Worth it to try both small and large. If you spend over a certain amount at one place (I can't remember...200E?) you get a tax refund back on your cc a couple months later if you apply for it.
Oooh, and check out the pharmacies! They have all kinds of really cool lotions and potions. I particularly like Roche Posay sunblock. Can't find it here in the US, and it's wonderful.
Take lots of pictures! I hope you have a great time!