Don't bring back any Cuban Pesos as they're not worth anything outside Fidel's "socialist paradise". You're right about the loony, you can exchange it there. Don't be too surprized when they tell you that a Cuban Peso is worth more than your dollar, the cuban economy is full of great surprises like this!
You can try to bring back more rum and and cigars that you're allowed too. A cute couple coming back from Cuba on their honeymoon won't get searched (unless you act like a prick when you go through customs). Also, don't say "hola" at canadian customs, they hate that.
Wich company are you flying with? Air Canada? Transat? Cubana?
American travel to Cuba is restricted by both US and Cuban authorities. You can still go via Mexico or someother country's airways.
For Americans its too much trouble unless you really have business in Cuba. Go to Mexico or Florida, ect for beaches and relaxation.
Thousands of Americans transit through Montréal on their way to or back from Cuba each years. They might get asked a few more questions that Canadians or Germans, but US tourists are welcome in Cuba, especially their money.