This is one of Toronto's best kept secrets. I take as many people as I can here. It's open every day (check their website for hours). There is a section for cacti, a tropical jungle that is sweltering even on the coldest winter day, a temperate zone, and more.
There's also an amazing playground for children (or adults) just outside. If you're looking at a playground, and don't think it's amazing, that's just the small one with one sad swing. Keep walking - you'll see it!
There's also a dog park on one side of the building.
If you've never been, you really must go!