Forget those designer boutiques and get on the metro to head to the Place d'Aligre for a fascinating market full of clothes, antiques and world goods (particularly great for African crafts).
There really is something more enjoyable about grabbing a bargain at the market than flashing the credit card down the Champs Elysees!
N.B. the market closes for business around 1pm (Place d'Aligre, 12e arrondissement)