Depending on your connecting flight times, 5.5 hours doesn;t give you a lot... expect a $45 cab fare + tip from Dorval to the Old Port and depending on traffic, 15-20 mins each way. That gives you 4.5 hours , and it's probably a good idea to be waiting at the gate at least an hour in advance of your flight, so now you're down to 3.5 hours (3 to be safe!) and almost $100 in taxi fare. You can grab a quick lunch and walk around the port for a while though, I suppose.