My tip is to visit the annual 'Braderie de Lille' - the biggest flea market in Europe, with stalls on over 100kms of pavements.
It is held over the first weekend in September in the centre of Lille. From Antiques to Junk, from Books to Pictures it is all there. The walk from Lille Europe to the market is short, some 500 yards so access is easy.
I discovered it by accident. Leaving the old station (Lille Flandres), after an ovenight trip from Italy, I headed for a café and stumbled across a small part of the market. I was hooked!