Coming out of Montreal to the Quebec/Ontario border, you are pretty much stuck on either Highway 20 or 40. Once you're into Ontario, stop at the first visitor centre and pick up an Ontario map - the official government one if you can. Then choose north or south to avoid Highway 401 - dull, fast and busy.
If you choose south you'll be following the St. Lawrence then Lake Ontario, not necessarily along the shoreline, but it's the slower, scenic route. It is Highway 2 you are looking for. I believe you can pick it up at Lancaster, the first exit west of the Quebec border on the 401 at county road 34. Follow it. You can go all the way to Niagara Falls on Hwy 2 and see lots of beautiful country, farms, factories, small towns, urban sprawl, suburbs, cities, more farms and beautiful country.
Continue on 2 from Kingston and you'll go through Napanee, Deseronto and Belleville. Going south from Belleville is Prince Edward County. Flat, but beautiful riding, farmland, restaurants and wineries.
If you want to see Toronto, continue on 2. It will lead you straight downtown and out the other side - at a snails pace and you'll have a sore clutch hand. Toronto is a traffic nightmare so consider coming into town late morning or early afternoon, and exiting town earlier in the day.
Continue following 2 until Burlington, then follow signs toward Niagara on the QEW (Queen Elizabeth Way - big messy highway). Once you are across the big bridge - the Burlington Skyway, look for exits that will lead you to the Niagara Parkway - it may also be called Lakeshore Road/county road 87 and change names to the Niagara Parkway at Niagara-on-the-Lake. The Parkway will dawdle you past farms, vineyards and small towns - lots of B&Bs if you want to stop - through Niagara-on-the-Lake, then up river toward the falls.
Tourists take the QEW straight to the falls. Smarter folks take the Parkway and see the river along the way and other places like Queenston Heights. There are quite a few places where we are quietly reminded that we kicked your butts in 1812.
The stately homes along the parkway are worth gazing at as you go by. As a bonus, there are very few stop signs or traffic lights on this route, but beware of the speed limits.
The Parkway will lead you right past the falls. Park your bike for a minute. Take a selfie with the falls in the background and continue on. The road past the falls for about twenty minutes is a nice ride, then you're basically on your way to Fort Erie/Buffalo. Alternatively you can turn back and cross at the Rainbow bridge over the falls.
If you prefer the more northern route, which used to be rather rough, but has become gentrified more recently, once you cross the Quebec/Ontario border, instead of going south at Lancaster, go north on county road 34, then turn left/west on 43 at Alexandria. 43 will lead you through some Canadian Shield country and through some nice towns like Smith's Falls (look for the big new government sanctioned grow-op for medical marihuana!!) and Perth at Highway 7.
Turn left on Highway 7 and follow it west through Sharbot Lake (actually you have to go a little south on regional 38 at Sharbot Lake to get into town, but it's nice place with a couple of good coffee spots). Continue following 7 west. As long as it's not a cottage weekend the traffic should be easy. Otherwise get used to following powerboats and sea-doos on trailers...slowly. It's rockier than the southern route, but the bonus is blueberries in season.
You'll go through little places like Madoc and Marmora, and then larger places like Peterborough (the lift locks are worth seeing). Highway 7 gets mixed in with some other highways around here, but generally head west toward Port Perry through Lake Scugog. Then 7 dips down toward the Toronto sprawl. Again, if you want to see Toronto, pick up Highway 48 south all the way down to Lake Ontario, and turn right on Highway 2.
If not, stay on 7 and go through endless awful suburbs and big box malls. Some people probably like that. If you want to zip through, get on Highway 407, which is an electronic toll road. They'll mail you a bill in a few months. You can decide whether or not to pay it
On this Northern/Highway 7 route, if you want to avoid the Toronto sprawl, just west of Port Perry, Highway 7 turns south with Highway 12. Don't turn. Go straight ahead toward Goodwood on 21 to Goodwood, which turns into 40. Turn right onto 48 or any other north/south road past it and then left onto 31 toward Newmarket. At Yonge Street, the worlds longest road, 31 becomes Highway 9. You'll be going through a little big box territory and small towns too.
Highway 9 follows the Holland Marsh along the south side, some of the most productive farmland in the world - leftover glacial lake deposits at a million dollars an acre.
Follow 9 toward Orangeville, then pick up 24/124 south. 124 turns southwest toward Guelph - good beer there and lots of farms. Guelph is a nice small city and a little confusing, but follow 124 in and then pick up Highway 6 south. Highway 6 will lead you to Highway 401 and you'll have to go one exit east at Morriston to pick up 6 south again.
6 south leads you to Hamilton - think of it as Pittsburgh in Canada. At Hamilton, take Highway 403 East to QEW South (or Niagara) which will let you pick up at the Burlington Skyway as described above and on to Niagara.
Of course, if you like superslab, stay on the 401 from Quebec all the way across until you see 403, then follow it to the QEW to Niagara Falls.
Oh - if you're taking the Highway 2 route along Lake Ontario, but you want to avoid Toronto, just after Bowmanville, turn right on county road 57 and go north to Enniskillen. Turn left onto road 3, which leads you into the suburban north end of Whitby at Brooklin, but then you can turn right on Highway 12 and go north. Turn left/west at road 21 toward Goodwood, as described above on the Highway 7 route.
There are plenty of other ways to do this, but taking Highways 2 or 7 gives you a full two days, or a pleasant three days, across south-central Ontario. Don't forget, you'll need a passport.