A flight plan is only required in the absence of other forms of notifying your flight. A "flight itinerary" with a responsible person is an acceptable alternative. If you have rented the plane, and are telling them where you are taking it, that should suffice.
I'm not as familiar with the rental outfits in the Montreal area (though I was there two weeks ago) so I will defer to others. There is a very convenient GA airport just west of Montreal called "Les Cedars", though the other places are fine too.
If you're bored, fly up to Mirabel. Ask if there is a landing fee (I don't know). If it's reasonable, land on a 12,000 foot runway, at an airport which is not at all busy.
A flight east just past Quebec city will take you to an island in the St. Lawrence, with a runway, and very quaint community. North of there, a flight up the Saugeny river is spectacular, though get a chart, and watch for wires, there's a span hundreds of feet above the water. Pay attention, and they're easy to see. Very rugged terrain - go only in good weather.
Be aware, "distances are farther than they appear at first glance". As Genghis told me: "England is where they think 100 miles is far, Canada is where they think 100 years is old". Plan your fuel stops.
In Quebec, you will find that French is very common in controlled airspace. The controllers will communicate in English no problem, but the other aircraft in the pattern may not. That will leave you having no sense of where anyone else is in the pattern.
Niagara Falls is a good flight, though several hours from Montreal. Toronot airspace is no problem if you follow the shoreline, and talk to the required controllers. The falls itself is closely regulated as to pattern and altitude for overflight, so don't stray.
Enjoy...