From Ottawa, hwy. 416 to hwy. 401.
The 1000 Island Parkway starts just west of Brockville Ontario, exit 685 off of Hwy 401. It runs west along the St. Lawrence River, for approx. 40 km / 25 miles, ending at the Town of Gananoque (Gan-An-Ock-Way) the “Gateway to the 1000 Islands.
From Gananoque, re-enter the 401 and continue west to Toronto.
Leaving Toronto, take hwy. 401 west to hwy. 403 to the QEW to Niagara Falls.
You could spend an entire week in Toronto and still have plenty to see and do !
Montreal should be 3 - 4 days, minimum. Ottawa 2 days. Niagara Falls requires a full 24 hours.
A suggestion to extend the scenic portion of your trip. The 401 is anything but scenic!
If you have time, (and as mentioned above) continue on Hwy 2 from Gananoque to Kingston, then take Hwy 33 along Lake Ontario to Prince Edward County, (Glenora Ferry is a nice treat). If you have time, stop in the County at a few wineries, visit the villages of Bloomfield and Wellington before going back up to the 401 at Belleville.
A diversion well worth it! You could do it relatively quickly, or spend a day in the County to really appreciate this gem of a place in Ontario.
Stay off the 400 series highways. Canada was built on rivers, so the old roads near the rivers are much more enjoyable than the concrete trucking canyons of the 400 series.
If you're going into Ottawa, stay away from the 417, take the 174 instead. Take the 19 instead of the 416.
Depending where you are leaving from in Montreal, aim for Beauharnois/132 or Oka/148 as they are both really cute.
Just a note from Montreal to Ottawa, you will to take the 40/401 until Hawksberry then it will actually be the 17 you will take for the more "senic" route, but it will be slower through towns. The 174 does not actuallt start until you are around Orleans (the suburb at the east end of Ottawa)
Another option to get to Toronto from Ottawa is to take Hwy 7.
Now many of these road that we call highways are actually one lane each way, except for the 400 series, and the 40 in Quebec
Nobody mentioned the hefty price tag of drop off fees for interprovincial drives. Renting a car in Montréal and dropping it in Niagara Falls will be expensive. Nowadays most large car rental companies are doing a Montréal to Toronto or vice versa free of dropping charge. Just check with all the big brands to find what suits you. Then drop the car in Toronto as you don't need one to visit the town and then rent again to get to N-F, checking again for no drop off fees. Of course if you return to Toronto to fly back home then there is no problem at all but if you were planning to drive to Buffalo (NY) and fly from there you are looking for a lot of trouble and money.
Also nobody mentioned the charming little town of Niagara-by-the-Lake by the scenic Niagara Parkway and may be stop at some wineries while en route.