The Kawartha Lakes are quite built up, so if you're looking for wilderness scenery, not the place to go. The walleye fishery is not the same as 10 years ago either. August would be tough.
You can check out the Martin River system that is north of Nipissing, Lake Temagami, Bark Lake (search for the one that is close to the town of Barry's Bay). Just a couple to consider to start with.