Are you there yet??
International Fireworks Festival 8s a must do and can be viewed free from the Jacques-Cartier bridge or by the clocktower in Old Port or if you like history etc. The Stewart Museum (an old British fort) in Parc Jean-Drapeau stays open late for fireworks watching. If you are an amusement park person the festival is actually held at Laronde.
OldPort has lots to see walking around...shops/vendors, restaurants and bars in cool old buildings, buskers and street festivals...depends what you are looking for.
Montreal blog
https://www.mtlblog.com always has lots of interesting ideas as well...