Hi Daniele,

congrats on planning your first Algonquin trip. What part of the Park are you thinking of visiting/where are you coming from?

Hey Daniele, 

You'll have a great time I'm sure! Generally speaking the deeper you get into the park, especially once you're over a longer portage or two, the fewer people you'll encounter. With two or three nights there's only so far you can get, especially if you're not experienced paddlers and will want to plan conservatively. 

So with only two or three nights, the best way to find seclusion is to start at an access point that's already somewhat off the beaten path. If you're willing to drive to the far North of the park, launching from Brent (access point 27, Cedar Lake) will get you away from the madness that is the Highway 60 corridor in August. You could work your way up the Petawawa to Catfish Lake for a couple of nights then return the same way, or travel from Catfish to the Nipissing and work your way down the river back to Cedar. 

Catfish in August won't be empty by any stretch of the imagination, but it will be far more peaceful than anything you could get to from more commonly used access points and is a beautiful lake. And if you travel back down via the Nipissing then that will give you good chances for wildlife viewing. The tradeoff is that this access point can be a very long drive depending on where you're coming from. From Toronto it's about a 6 hour drive.  It sounds like you're going to need to rent everything, so you'll need to arrange that in advance if you want to use this access point:

http://algonquinoutfitters.com/store-location/brent/

Algonquin Outfitters has been operating the Brent Store since 1982. Outfitting, canoe rentals and supplies for camping trips are all available at Brent. Advance reservations (minimum four weeks) are highly recommended for any rental or outfitting package.

I'd look at Rock lake access to Pen Lake for 2 nights. Decent paddle times, small portage and nice sites. It's the lake that started it off for me. From Pen you can do day trips to other lakes or just explore Pen itself. 
That Cedar loop Uppa suggested is a beauty for sure but a bit more of a challenge (i.e. Unicorn Hill) which may be harder for first timers to the Park. 

My advice would be to choose a well travelled route, close to highway 60, but camp on small lakes with few campsites. If it's your first overnight canoe trip, you should stick to easy portages (not necessarily short, just well maintained and relatively flat) and well travelled areas in case of an emergency. The outfitter options near highway 60 will also be extra convenient when it comes to renting.

As someone already mentioned, you won't be able to get to remote areas with only 2 or 3 nights. Your best bet to cover more ground is to end somewhere separate than where you start, and have a shuttle pick you up (outfitters offer this for a fair cost).

I'd recommend something like...

1. Smoke Lake to Cache Lake loop (through Ragged, Big Porc, Bonnechere, Kirkwood, Harness, Head)

2. Rock Lake loop back to Rock Lake (through Pen, Welcome, Harry, Louisa, back to Rock)

3. Source Lake to Cannisbay loop (through Bruce, Owl, Raven, Linda, Polly)

They're all popular loops and some of the lakes have campsites that are spread out, so you get the sense of privacy and remoteness. But they're all well travelled and if you run into any issues, you will be able to find someone to help.

I second the Rock-Pen route! If you're novice paddlers, I think you'd want to stay away from the bigger lakes like Smoke or Cedar- they can get a little ugly when the wind picks up. You could spend the first night on Rock on one of the island sites and explore around the lake for the day, there's lots of stuff to see like the Booth estate, the old railway, petroglyphs on the rocks, and lots of beaches, cliffs, bays, and marshes to check out.
Move down to Pen the second day over one slowly rising portage (partially on a boardwalk) fill up your water bottles with COOOLD fresh water from the spring, at the halfway point, and put in on a sand beach with a dock. Best site is about 1/2 way down the lake on the east side on the point, gravel landing, nice big FLAT site, but best of all is the beach on the backside of the point It's huge and usually is in better shape than the "public" beaches In the organized campgrounds.
Get up early on day three (easier paddling before the wind picks up) head back to Rock, hit the showers, make the prerequisite stop at the visitor's center, and if heading to Ottawa, Stop for a late lunch at the Mad Musher for one of the best burgers in the area!