If you have never visited Niagara falls(Canadian side) you might want to do it on your drive to Toronto.On your drive from Toronto Montreal;short visit of Kingston and drive the Thousand islands parkway. Take the one hour boat ride from Ivy Lea.They round the island where Boldt's castle is situated. We usually travel after US Labour day. We had a wonderful stay on a lake in Algonquin park in a great motel called Lakeside inn. They have canoes free for use and the park was wonderful due to the splendid fall colours.There are many lakes in the enormous park and I have heard that Killarney lodge has lodging facilities.I searched on Google and found the following site:www.algonquinecolodge.com which might be what you're looking for.Paul

Briefly:
Toronto:
- St. Lawrence Market downtown - closed Mondays, most vibrant on Saturdays
- Harbourfront - on summer weekends Cultural and Music Festivals, craft market, outdoor dining, sightseeing cruises, boardwalk to stroll and watch the sailboats
- AGO (Art Gallery of Ontario) - first class
- ROM (Royal Ontario Museum) - also first class including recent spectacular Dinosaur Gallery addition, Geology Exhibit, Human History, etc.
- Just northwest of Toronto - McMichael Gallery - famous Canadian early 20th century impressionists (Group of Seven)painted landscapes of Canada's north and west. Also Aboriginal art collection.
- Huge variety of Restaurants

Montreal:
- Old Montreal Historic District - very charming, cobblestone streets, usually Buskers and musicians in summer, many patios, bistros, cafes, galleries and shops, etc.
- check whether Cirque de Soleil is in town.
- Excellent Botanical Gardens
- Biodome - indoor zoo divided into ecological zones, just across from the Botanical Gardens
- Montreal Museum of Fine Art
- Fabulous dining all over Montreal - high end to (famous) deli and everything in between (in both price and type of cooking)!

Lakes In Between:
- Algonquin Provincial Park (mentioned by tovarich) is one of our favourite outdoor getaways. Huge wilderness park, rugged forested hills and many lakes. The main highway traverses the south corner and offers access to excellent short and longer hiking trails, canoeing, kayaking, wildlife watching (famous for moose sightings and wolf howls) and a very good Visitors Centre/Museum. Three older lodges in the park, lots of camping (but very popular, book ahead) and a wide mix of accommodations just outside the park including private cottages for rent. Google Dwight, Oxtongue Lake and Huntsville Ontario.
- The Muskokas north of Toronto are popular weekend and vacation destinations - lovely lakes, great boating, but lots of cottages and summer homes (some are mansions) - again rentals are available, lots of online sites. There are also a number of resorts sprinkled around. Check out http://www.ontariotravel.net/TCISSegmentsWeb/main.portal?language=EN for other ideas.
- The best summer cottage rentals can book up well in advance, up to a year in some cases (we've already lost out on our choice for next year because of this - lots of repeat customers who are usually given priority) so if that is your plan, you might want to start working on it asap.
Again this a very brief overview, detail could be added in response to more specific questions as you narrow things down.

Great ideas - thanks so much! Off to research.

Montreal:
If your visitors are Dutch, they probably bicycle. Montreal has the best network of bike trails this side of the Atlantic. They are safe, and go everywhere including the very popular Lachine Canal trail. FYI, the entire provence of Quebec has the best network of bike trails in the world.
Montreal has two very extensive farmers markets - Jean Talon, is the largest and is near the Italian section. Atwater is also quite large and can be accessed from the Lachine Canal bike trail.
Montreal always has some type of festival going on - jazz fest, Francophile fest, fireworks competitions, etc,etc.
You won't have any trouble finding good food in Montreal. The plateau neighborhood is noted for inexpensive, ethnic, byob restaurants with outdoor terraces.
Go to the bonjurquebec.com website and request a free visitors guide to Montreal.

Love this idea for a trip.

I would do 3 days in Montreal, 7 days at a cottage rental north of Toronto (check out Muskoka, Haliburton and Kawarthas as general areas for nice lakefront properties - and as suggested above book early), 4 days in Toronto (with a day trip to the Falls).

www.vrbo.com has cottage properies and you can search Ontario and the areas I mentioned above to narrow it down.

Also you can google cottage rental sites. There are several for Ontario cottages.

Here are a couple reputable ones:
http://www.ontariocottagerental.com/
http://www.cottagerental.com/