I am creating a trip for the summer of 2021 from Markham to PEI and wondered if anyone can suggest and nice roads to take on the way.
That sounds like a really nice trip.
I didn't make that trip myself before but if I were to do it, I would start with the overview map (shown) and base my route on that.
The trip builder feature of this site allows you create a new route on a map overlayed with this routes (to make it a lot easier) but if you prefer not to become a sponsor, you can simply view all that route data here:
Good luck with your trip!
All the Best
Here is the image I wanted to post in the previous comment but was not able to because of a bug (which has just now been fixed)
Years ago I rode from Toronto to ride the Cabot Trail in Nova Scotia. My suggestion would be to ride to Lake Placid on day 1, stop at Ben and Jerry's on day 2 and then continue through to the coast before heading north. Watch out for Moose on the way through New Brunswick.