Canada's Wonderland just opened what they say is the biggest rollercoaster in North America (or something like that). They call it the 'Behemoth', I think. It looked pretty big during the construction, but I've never been on it - I'll probably hit up canada's wonderland over the summer, but ticket prices are incredibly expensive, and keep on rising.

It's about a three hour drive north from the Niagara Falls border (and just on the outskirts of Toronto (Vaughan) - looks like you're near that area anyway.