I canoed from Cambridge to Brantford (a four-hour trip) in Spring 2007, which I was told was the best season to canoe. The weather could get too hot and the water runs much more slowly in the summer.
Canoe rental fees are generally reasonable and the trip is worth the price. Two rental places I know are Heritage River (which I used and was very satisfied with) and Canoeing the Grand (which could be cheaper for a shorter trip). Remember to bring sunscreen; large sealable bags for camera and cellphone; plastic bags, or even better buckets with lids for extra dry clothes and shoes.