Since you know about Mont Tremblant, you know that there is plenty of wilderness once you get past the resorts. Their web site can tell you if you need to reserve camping locations.
In the Charlevoix area, the Parc National des Grands Jardins, just north-west of Baie Saint Paul has nice trails and is a bit more isolated than Tremblant, although that may not be true at this time of year. The hike up to the top of Mont Lac des Cygnes (spelling??) is fun and gives nice views. Parc national des Hautes-Gorges-de-la-Rivière-Malbaie, just north-west of Malbaie is a beautiful spot, lots of trails. Both those areas are close to major towns, easy to find good food and shelter. UNless you get unlucky and hit a heat wave, temperatures will be cool(er) too. It's a shame you're not going in autumn, colours turn towards the end of September.