I rode around Eastern Townships few years ago, and it's a really nice area, but mostly on roads. There's Yamaska Park with dedicated paths, which is really nice.
You could also ride Le Petit Train du Nord http://www.laurentians.com/parclineaire/ (we just did that few days ago). It's a rail to trail, so grades are only about 2-3%, which would work for you. The north part is paved, the south part is crushed stone (which we found really quite treacherous to ride on a fully loaded bike).