Are you looking for suggestions, or trip reports? Some suggestions:

Ottawa to Montreal, or Montreal to Quebec City via highway 138: Very flat, very fast from west to east unless you're unlucky and get easterly winds.

P'tit train du Nord trail: 200km one-way. There are shuttle services that can bring you to the northen point.

Véloroute des Bleuets: 256km, easy with lots of services.

Véloroute du Fjord: Not an official route. About 330km on highway 172 and 170 on either side of the Saguenay fjord. I'd suggest taking 5 days to ride to the fjord. I got some pictures here.