Since nobody answered you and I have an intimate relationship with the brown trout in the Diable river, I figured I'd answer you:
The Diable river starts its journey far north of the Mont Tremblant provincial park, but gets exceptionally beautiful towards its exit from the park's land and starts through a 5-6km fly-fishing only sector managed by the club called Les EnDiables.
Membership to the club is not mandatory to fish the waters, however you can learn alot from its club members and enjoy some of the events they put on:
They do stock these waters with healthy brown trout (as this thread demonstrates very nicely), but there are natives in the system and I've heard also brookies/speckies, but never caught any of them.
There is also one spot that I ran into something that could only have been either a pike or an angry beaver, which smashed my fly drifting on the opposite bank just before the last rays of sunlight disappeared once...
Anyways, the river is most definitely very beautiful, however its quite challenging: to fish and to wade. Large round boulders abound and currently can be a little swift in spots. Regardless, this is excellent trout water with very easy access. The club's website has maps.
If anyone was interested in heading up there with me in the late summertime, I'd be happy to "guide" for free. (I get to fish too, though :P )
I know most of the nice spots and also the LaCachee river triburairy is really nice too.
I drive through Lachute quite often, so picking you up would be easy.