Like you I went to Quebec City, only this was a few years ago. I took the same trip to the Plain of Abraham, visited the extensive network of fortification and also went to see the water falls where The British had landed. There are still quite a few remnants of the French and/or British defense system.
The battle field diorama you talk about is a brief history of the last days of French Canada, using one large diorama of the city, its surroundings as well as the troops deployed on the plain of Abraham. They use light to get your attention focused on each different period. It's nicely done, although in a way quaintly antiquated... I really loved Quebec and would not mind living there for a while.
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.