Lived in Québec for a year, and have gone back from time to time.
My favourite restaurant there is Aux Anciens Canadiens - Quebec country cooking, in the heart of the Old City. It’s right opposite the French consulate, the building where Montcalm died, and the Duke of Kent (Queen Victoria’s father) kept his French-Canadian mistress (not both events at the same time, of course).
Another good restaurant is the Auberge Louis-Hébert on the Grande Allée.
The Upper Town, inside the walls, is one of the oldest cities in North America. I’ve spent hours just wandering around. There’s also the old Lower Town, beneath the walls - well worth an afternoon of just wandering.
The National Assembly is a beautiful building and well worth a tour.
What time of year are they going? It may be a bit late for the fall colours, but I agree with Larry Mudd’s comments about driving around Île d’Orléans and out in the countryside.
They usually illuminate the Grande Allée around Christmas time - that’s the main road leading to Porte St. Louis and the Château Frontenac - the first time I took Mrs Piper there, I timed it to arrive in the evening, when the snow was just falling, and we drove up Grande Allée - she loved it! That time, we stayed at the Gîte de Paris, in the heart of the Old City - great little B&B.