When are you going and what are you fishing for? I've stayed at Pog lake, right beside Lake of two rivers, there are smallmouth and lake trout in that lake(two rivers). Other lakes around also have rainbows, brook trout, or splake. There's also a nice swimming hole just upstream the lake at the falls, and also a few trails around with nice lookout points if you're into that.
In early spring when trout are up high just about every lake is easily fly fished in Algonquin. Streamers of buggers stripped have worked well. There are also the odd stream and river that holds good fish although would require a walk or paddle to reach. World Class trout fishing if you hit it right.
When I was in my late teens/early twenties, I used to camp and canoe trip in Algonquin a lot. Even then I found the park to be on the crowded side, today I'd image it would be by reservation only. Even though I enjoyed Algonquin a lot, I can think of a lot of other places to go that are less crowded and more affordable to spend my time at. In my opinion, Algonquin has become a zoo. When I plan to go camping or take a canoe trip, I'd rather spend my time in a more relaxed environment otherwise I'd feel like I'm still in the city..