It's always good to check out what's happening in Fondation Cartier. They have lots of really good exhibitions of modern art, and the gallery itself is very beautiful. Perfect for seeing works you already know as well as discovering new artists and inspirations.
There's a few secondhand clothes stores in Paris, but always immaculately selected, expensive and fashionable. Guerrisol however is a huge, dirt cheap jumble sale. Stock seems to rotate daily in each of their four stores, and prices fluctuate on the hour. Sifting through endless rails of synthetic jackets at €3 can suddenly reveal a hidden gem.