Sounds like the potential for a great trip!
If I were to do it, I'd be looking hard at heading northeast and pick up a ferry to the north shore of the St. Lawrence. Never been further east than Tadoussac, but it is a gorgeous route back along through Charleviox to Quebec city and very popular with bikes. There is a ferry at least as far east as Matane and also further west at or near Rimouski. Take a break to ride the ferry that goes up the Saguenay, or time permiting ride to Chicoutimi. Discover that you don't have to go quite as far east as Grosse Morne in Canada to see a fjord. We even saw whales in there a few years ago.
From Quebec to Montreal, someone else will have to contribute. Drove it once staying off the highway and following the river, but to be honest, I don't remember much of it. I think we ended up heading north a fair ways to find a place to stay and found the route more enjoyable "up there", wherever we were. Might be worth dodging down into the "Eastern Townships", Centre-du-Québec", for that stretch if time allows.
The underside of lake Ontario isn't exactly jaw-dropping if you are hugging the shore. We did a loop around the lake a year or two ago. Stateside, I think you need to be further south to find the interesting routes. Something I plan to explore a bit myself this season. Someone else might be able to elaborate on this area if you plan on cutting thru that side to come back in at Niagara Falls.
North Shore of Lake Ontario, back in to Toronto can be fun if you stay off Hwy 401 as much as possible. I will usually stick to Hwy #2. Not very fast and goes through all the little towns that developed as Ontario was being populated. Kind of a history lesson as you go along and generally scenic with good riding. Also lots of B&B's to stay in as required. At Kingston, lose #2 in favour of #33 "Loyalist Parkway as you head west. Then you get to take the short, (free), ferry to cut through Prince Edward County. Some great food to be enjoyed here, and camping at Sandbanks Provincial Park.
Pick up #2 again either at Trenton or Brighton. (Trenton if you are interested in the air museum at the base where the recently restored, only complete example of a Handley Page Halifax exists. The U.K. has one in York, but I'm told it was cobbled together from a few machines and would probaly shake apart if the engines were started...but that's another topic.)
Eventually , around Port Hope, the benefit of staying south of the 401 is lost. Either cave in to the urge to just get into the city & see your son, or you have to head north of the highway and follow your nose in the rest of the way.
Check this link for some more route suggestions once you get into Ontario: http://bikeroads.atspace.com/
Hope this helps a bit, and again, "two thumbs up" for the north shore of the St. Lawrence!