Ontario/Quebec, have you seen fall foliage in September?
 

My partner and I will be arriving in Quebec City September 12, taking a train to Montreal, and departing Toronto September 23rd.

Could we rent a car for the day and see fall foliage somewhere this early in the year?

A hour-and-half drive one-way from one of these cities will be doable.

The foliage does not change colours until well into October. It sometimes depends on the weather, but the warmer the fall, the later it happens.
An hour drive north of Toronto will get you to lake Simcoe.(east side). You can have a nice drive ( walk) by the lake, visit the Briars resort ( golf course)for lunch etc...

Danon, so there is no major foliage change at that time in the mountains north of these cities?

North of Ottawa and Montreal, in the Gatineau Hills and Laurentians, the leaves are in full colour starting in the last week in September. It's a bit earlier (perhaps a week or so) north of Quebec City. North of Toronto, it's a bit later--first week of October or so.

The leaves will be starting to turn while you are here, but there's not likely to be anything spectacular before you leave.

There are no mountains north of Toronto.
We have a place an hour north of the city - it is almost a suburb.
I would say mid-Oct. for southern Ontario, but could be earlier or later depending on the weather. Two years ago we had guests from Ireland whose main reason for coming in Oct. was to see "the colours'.
It was a warm fall and nothing much happened until the 3rd week in Oct.
Regardless, the fall can be a beautiful time to drive out of town on a sunny, cool day with blue sky by some lake or in the hills.

P.S. lakes in Ontario, hills north of Montreal.

I am aware of the Laurentians north of Montreal. Are there mountains north of Quebec?