Opeongo River
Due to the water levels, this river can only be run in the spring and it's for intermediate paddlers and above. It's a great 22km day trip filled with whitewater. The Opeongo and the Upper Mad can be run in a weekend, but with two long days. "Base-camping" a motel makes it an easier go.
You'll need to get an Algonquin day pass for your car since it starts in the park.
Best resource: If you're new to the Madawaska, you really should pick up George Drought's Madawaska and Opeongo Guide Book. It's got everything you need for this river.