It's been close to 20 years since I was in Montreal - I figured one of the eastern Canadians might have offered some info. Montreal is a city I would love to visit again with more time.
There are several festivals, 'Just For Laughs' comedy fest, the 'Jazz Festival', 'International Music Festival' - timing of the events I am unsure of.
Mont-Royal is the viewpoint referred to by Wos.
Notre-Dame Basilica is one of many fantastic old cathedrals.
Old Montreal is full of historic buildings and a few days worth of sight seeing itself.
The Botanical gardens, were awesome, the port along the St Laurent is worth seeing - although it's changed quite a bit since I was there.
Have a look here for more.
http://www.infomontreal.com/attraction_en.asp
There is an event calendar here:
http://www.vieux.montreal.qc.ca/calend/eng/req_cala.htm