Hi DDW,
I've never done the St Lawrence above the 1000 Islands, but of course Montreal and Quebec are both wonderful cities to visit... My understanding is that Saguenay River is not to be missed, sounds like a spectacular place, perhaps giving you a bit of a taste of what the fjords of Newfoundland are like...
I really loved the Isles de Madeleine in the Gulf of St. Lawrence north of PEI... Very interesting place, VERY French, very few other cruising boats there, you'll be constrained a bit by your draft - especially around Havre Aubert - but you'll still be able to see a lot... I loved that place...
From there, you could sail around the top of Cape Breton, stopping in places like Bay St. Lawrence and Ingonish, then down into the Bras 'd Or lakes... They are definitely not to be missed, a wonderful area...
David knows the NS coast better than anyone here, but I'd certainly second his recommendations on the Lunenberg/Mahone Bay area... Port Mouton is another of my favorites, the beach and dunes there are somewhat unique by NS standards...
Unlikely you'd have the time, but a cruise up the St. John River on the other side of Fundy is a wonderful trip... But perhaps you could manage to get into Passamoquoddy Bay and St. Andrews, another delightful spot... I also really like the area around Grand Manan/Campobello/Eastport, little visited by cruising boats, but you'll feel welcome there...
Of course, don't miss Roque Island/Mistake Island area around Jonesport, some of the best Maine has to offer east of Mt Desert...
the pic is from the Magdalens, cool place...
best regards,
Jon