OK, so basically you're looking to fill the five days between leaving Montreal and arriving in Niagara. I see two basic options:
1) Swing south-southeast through Vermont/New Hampshire, then turn west and head to the Finger Lakes on your way to Niagara.
2) Head due south to the Adirondacks, then southwest to the Finger Lakes.
Lake Champlain gets in the way, so you have to decide whether you want to pass east or west. Option 1 is longer distance, but nicer scenery and has more interesting towns to explore; it would be my choice if I was planning a road trip. Via the shorter option 2, you're looking at maybe 600+ miles on the road, whereas option 2 is 800-1000+, depending on your route. Assuming you're not looking to just hammer across the interstates (which won't be an option in some areas), and you're going to keep moving rather than stopping in one place, you're talking about anywhere from 3 to 6 hours in the car per day.