There are two excellent dioramas in Quebec City. The one Philippe describes is in the private museum across from the Chateau Frontenac. It is excellent with the light show that goes along with it.
A little more out of the way is the artillery musuem that is just inside the walls of the city. It is run by the Federal Government. In it they have a diorama of the city that is huge, and clearly shows the fortifications post War of 1812. It was created in about 1820 as a model to show how the fortifications should have been expanded. After completion it was shipped to Britain to the War Office. After a century or so sitting in Britain it was shipped back to Canada and put on display. The scale is a little less than 1/72nd, but it shows the entire city!
Quebec City is a UN designated world heritage site and well worth the visit. And don't worry if you don't speak French, the city is very tourist/user friendly.