Isn't there a giant flea market?
If you time - get lost. Seriously take a phone with GPS so you can get back and just wander around the older parts of the city and poke your nose into courtyards and odd corners.
You don't need a GPS in Paris, seriously. It's a small city.
If you get lost, you just ask somebody the way to the nearest Metro station and use the maps of the network and the neighbourhood to get to wherever you want.
I'm sorry but my knowledge of the french language is not much more than " bonjour, je m'appelle Elodie, je ne parlais pas francais, je n'ai acune idee, parlez vous anglais?"
i will need gps for geocaching anyway.