Hi Nick, I second Rug's recommendation for Algonquin Park. It is about 300km north of Toronto's Airport. It is one of my favourite places to camp in Ontario.
Check out Algonquin Outfitters for any equipment rental needs you may have. I've never rented from them personally, however I see people in their canoes regularly in the park.
I go with a group of guys in the first few weeks of May to go trout fishing every year. Generally that is just before the bugs begin to eat people. If you can't go before the bugs get bad, I'd go anytime in September or October and you should be able to expect temperatures well above 10°C at night (worst case).
You can get maps of the park from Ontario Parks website.