Gaspésie loop and closing via highway 299 makes a great week-long trip. There are many more week-long tour to be made in Quebec, some in loops, some not, but the Gaspé loop would be my first choice. Here's thread about it
Cape Breton is great as well. I've only followed the northern coast from the west to North Sydney. I'd like to go again to ride on the southern side.
I made a short stint into PEI and it was nice but I'd choose the above two before.
The west coast of Newfoundland is awesome but getting there and back is a problem.