Head down to the waterfront and take the Martin Goodman trail west. This bike tour is signed the entire way and becomes the waterfront trail, mostly on bike paths but there are some road sections. I have never found traffic to be an issue on this route.
In Niagara-on-the Lake follow the Waterfront Trail to the end at Fort George. Then do a right turn on the wonderful Niagara Recreation Trail which is a paved bike route all the way to Niagara Falls.
It is a wonderful route, have a great time cycling.