The Montreal botanical gardens are rapidly becoming one of my favorite places in the city to visit. It's one part of the Montreal Space for Life which also includes the Biodome, Insectarium and the Planetarium. It's a 75 hectare National Historic Site of Canada and is considered one of the most important botanical gardens in the world. Just a short walk from the Pie-IX metro station it's open year round.
Containing 22,000 species spread throughout 10 greenhouses and 30 gardens many of which are located outside the place is huge. Montrealers can check out the outside stuff for free.
There're special events and exhibitions all year and many of them are actually quite good. Walk amongst thousands of butterflies in the "Butterflies Go Free" exhibit held annually from February to April. Gardeners will be interested in the annual plant sale held in May. It's worth checking out their website to see all the stuff going on.
Open all year, continually changing, educational and family friendly, the only real downside is that tickets can be steep. Starting at $15 for Quebecers, special events sometimes cost a bit more. The gardens are big enough though that it will take hours to get through just a portion. The International Mosaicultures event currently on until the end of September 2013 runs $25 but the ticket is actually valid for two visits. Of course, the outside bits won't cost you a dime.
It's a fun and educational outing that breaks up the monotony of city life and breaks you out of the rut of cocktails, live music, performance art hipster routine.