Lille?
 

A day trip from Paris or a longer stay?

Day trip.

We stayed overnight a few years ago. Too much of it was leveled in 2 world wars. The old/interesting part is smaller than you might think. Have oysters at the famous oyster bar.

Trains go there frequently for the 1 hr trip.

Stu Dudley

Definitely stay overnight. And don't limit yourself to just Lille. The metro can take you to Roubaix and its fantastic La Piscine museum.

Overnight, at least.

Lille is a cool city with great beer culture and excellent restaurants. Some good museums too. I think it's worth a day or two.

Still one day if we include the Mallet-Stevens villa and La Piscine de Roubaix?

If you don't mind being totally exhausted. After all, the trains from Paris only take one hour and they are plentiful. (Hint: you will be happier with a train to Lille-Flandres rather than Lille-Europe, but it is not really important since the stations are only 400 meters apart.)