Barron Canyon
The Barron Canyon contains Algonquin Park's largest and most impressive rock features. As its name suggests, it's a canyon—it has some big walls made of exposed granite. Starting at the Achray put-in, it's a beautiful almost entirely flatwater experience. You'll paddle through both beautiful lakes and charming river sections. The tiny bit of moving water can be portaged if you're paddling a fancy-light boat, which you might need since there are several portages on this trip.
It can be done as a there and back again trip, or as a one-way trip with a shuttle.