There area actually 2 routes you can take to Niagara Falls going around Lake Superior and Lake Huron/Georgian Bay. One takes you down the Trans Canada #17 to #69 in Sudbury and this takes you straight down thru the heart of Toronto and then you will wrap around the edge of Lake Ontario to Niagara. This route is quicker if you are driving off hours, but can have a lot of traffic if you go during rush hour (cottage country weekend traffic or Toronto AM/PM rush hour). The alternate route is to go on to Manitoulin Island and take the ChiChimon ferry across Georgian Bay to Tobermory and take #6 all the way down to Niagara. This is much less urban and have Bruce National Park and Fathom Five National Park in the Tobermory area. This will take longer due to the ferry but if you want to see more animals and less people, I would favor this route.
For the top half of the trip it is worth stopping at Sleeping Giant Provincial Park near Thunder Bay, Lake Superior PP north of Sault Ste. Marie and Killarney PP (if you go around the Toronto route).
