It is not too hot, not too cold, you can wear a hoodie or a short sleeved shirt (or both), and the chance of rain is minimal.
The cooler temperature means you can cycle harder and faster without overheating.
Visually the scenery becomes quite brilliant too, so take your camera with you as you cycle through pretty neighbourhoods in Toronto - or take the bicycle with you as you visit locations near Toronto.
One of my favourite places is Hilton Falls near Milton. It has both hiking and bicycle trails. And with the waterfalls as part of the scenery it makes for a fantastic place to stop and enjoy the wonder of nature.
Hilton Falls is one of the few waterfalls in Ontario which you can walk around behind the waterfalls and then plunge your arm into it. The Hilton Falls Conservation Area has multiple bicycle trails of varying intensity. More than enough to challenge experienced cyclists - and plenty for fun for people new to cycling in the great outdoors.
I am already planning another trip back there in October when the leaves will be different colours.
There are many other fun locations to go cycling in the region around Toronto.
Things to take with you:
Camera / cellphone
Water / Sports Drink
Umbrella (not for the rain, for walking around behind the waterfalls - trust me on this one)
Depending on where you are going you might also wish to take a map with out. A proper paper map is easier to read than your phone app. So if you are going on a bicycle / wine tour of the Niagara wineries, take a map with you - and don't drink and ride!