My brother lives in Montreal so I've been a few times.
Honestly, nothing stands out as a must see (well, maybe Jean Talon market), but there are some interesting things to do. Beyond looking at a list on Trip Advisor, I'd recommend looking up some eclectic neighbourhoods outside of Old Montreal and downtown, and wandering a bit. But do make sure you spend time in Old Montreal.
If you're looking for food I'd highly recommend Au Pied de Cochon (make reservations), but Montreal is also one of the dining capitals of North America so you have options.
Oh and the Musée des beaux-arts de Montréal (Fine Arts) is free, and pretty decent.
