Old Montréal is very easy to get to, and hard to leave. As much as Tourisme Québec would like to tell you there are 6000+ parking places, I have yet to find one. The subway is definetly your best bet. It is a fairly small area (approx 3/4 square mile), but it is loaded with things to see and do.
The first thing we always to do is grab a carriage ride in from of the Palais de la Justice. (You can't miss it -- giant building with huge columns down the front.) The carriage rides aren't cheap (C$40 per 30 minutes, or C$65 per hour), but given that it's not an expense that comes up too often . . .
From there, you have plenty of things to choose from. The streets are loaded with small cafés and restaurants, little shops, etc. This is more of a spot to go for dessert though. Things are fairly expensive and dinner can easily run you upwards of C$100 for two people, long before you even look at the wine list.