I take some pride in finding this place before anyone I knew was aware of this place a couple years ago, when I had just moved into town. Having moved from BC, I was desperate to find nature escapes from the city.
There are a couple buildings, so you have access to nice clean washrooms and vending machines. There is a small art gallery too. What's really great is they do a tonne of summer camps, keeping kids active and learning about gardening. They even offer programs for people with disabilities. Moreover, I've seen events on SNAP where people meet up here for activities such as birdwatching. Of course there is the terrace garden.
My favourite thing about it though is that it provides access to the Culham trail, which follows the Credit River. It's a continuous trail beginning roughly near the 403 and goes at least to Erindale Park (near Dundas). It is completely gorgeous and peaceful along the river valley - a true escape from the city. Once summer reaches, it's incredibly lush. Anyone I've brought to the trail had their expectations far exceeded.
Right near the top of my favourite places in urban GTA.