It really depends what you want to do

If you think you will have energy to walk around a bit- no need to pay for an expensive taxi- there is a new bus service from the airport which goes directly downtown- it runs all the time. cost is only 7 dollars- but it is coins only.
here is the info:
http://www.stm.info/english/info/a-747.htm

The schedule says it takes 30 minutes into town but if you double that for rush hour traffic (worst case) that still leaves you with 6 hours in town and you can be back 2 hours early for your flight

I would get off at stop #4 you should be right near the main Montreal Tourist information which is at Peel & St-Catherine. This is where many of the city tours leave from. If you dont want a city tour I would walk along St-Catherine which has lots of stores- and look out for Crescent street for bars/restaurants.

There is a lot to do but it really depends what you want.