Quebec is an amazing city. i have fond memories of the 3 days i spent there. the city was founded in 1608 by a Frenchman; Samuel de champlain. there aren't many buildings in quebec, still existing, from the 1600s, but Quebec has a wealth of buildings from the 1700s and on.
most of the old town has been taken up by museums, cafes, shops, and government buildings. not too many people live in the old town.
one of the most interesting things I found, is the contrast between the upper and lower towns; they are really quite different. the upper town has more grand/monumental architecture, while the lower town is less spectacular, but has older buildings. i found that i enjoyed walking around the lower town more then the upper town, the culture soaks in more in the lower town.
the walls which encircle Quebec are famous (they are the only ones in N. American north of Mexico), and delight to walk along. cannons are placed at various intervals along the wall. the wall walk was one of the highlights of Quebec for me.
the chateau Frontenac was built in the 1890s, one of Canada's great railway hotels. this is a magnificent building, easily the most beautiful building in Quebec; and it dominates the town. the Chateau Frontenac can be seen from almost anywhere in quebec.
i recommend going across the river to Levis; the views of Quebec are incredible from there.
Practical Info: Quebec is a 9 hour drive from Toronto, and a 3 hour drive from montrael.