I’m a native. All the above I would echo. To add, bearing in mind a 13YO, Alice Délice (kitchen gadgets), Nature & Découvertes (hard to define, kind of homewares, kind of outdoors activity stuff), Le Chat Bleu (chocolate shop, tiny and easy to miss, don’t miss!), Casa, Yves Rocher, maybe even Petit Bateau? Oh, little Dr Martens shop, hidden away. All those could be fun. Or turn out expensive!
The zoo, citadel, and then Jardin Vauban opposite are worth a visit definitely if time allows. And I love La Treille, the see-through facade is crazy. Palais des Beaux-Arts, central and good. There’s a Modern Art museum in Villeneuve d’Ascq, looks miles away but is easy to get to on the metro. The metro is driverless so sit right at the front if you take it, it’s fun.
Shopping wise, obviously Euralille which is now a Westfield, but there are several small shopping galleries around the city centre, Le Printemps department store is good, also the big FNAC and Le Furet du Nord.
Food - I agree Grand Place is a bit watered down, obvious reasons - catering to tourists - but it isn’t bad for a safe introduction to Ch’ti cuisine, and definitely has its use for things like a quick, cheap breakfast. Wazemmes on market days is a great place to get a tasty lunch from a stall. Also, I love Le Broc, which is a cheese restaurant. I agree Vieux Lille is a better place to go for a good, solid dinner though!