Citadelle & Jardin Vauban
The citadel is situated to the north west of the centre (on walking distance) and owned by the army. It's surrounded by a nice park where you can go for a quiet walk. At the park you'll find a zoo, a puppet theatre and boats for rent.
Address: Avenue du 42eme, Regiment d'Infantrie

Notre Dame de la Treille
The first stone was laid in 1854 and this neo-Gothic cathedral was finally completed in 1999. Notice the colorful circular window above the main entrance.
Address: Place Gilleson

Palais des Beaux Arts
After the Louvre this is France's most important museum of Fine Arts. In this beautiful 19th century building you'll find paintings from European painters as Rubens and Picasso. The inner courtyard has a glass wall in which the facade of the museum reflects.
Address: Place de la République
http://www.on-frame.com/en/lille/index.htm