Rail
With Montreal well connected by Viarail to Toronto (7 daily; 5hrs) and Ottawa (6 daily; 2hrs) you might be tempted to visit by train.
In any case Viarail's services are certainly handy for a side trip to Quebec City (5 daily; 3.5hrs).
There's also a daily service to Montreal from New York City (1 daily; 10hrs) with Amtrak.
All these services arrive and leave from Montreal’s usefully central Montreal Central Station.