your profile doesn't say how old you are, but since it's your first trip, your knees are probably much younger than mine.....unless you get some serious saddle time in before September, you're going to find 3 weeks of constant riding physically tiring. If you want stay in Canada, I highly recommend the maritimes, especially the Cabot trail (speaking from experience - sight see or ride, don't do both or you may end up on the wrong side of the road in a twisty, plan to stop at the overlooks, ride it clockwise or both if you've got the time!).
As Ryan said, 400 miles/day on back roads is alot (depending on just how twisty the roads you pick), if you're super-slabbing it (major highway), 1000 miles is doable, but the boredom will make you insane.
You can't go wrong with anywhere around the white mountains in New Hampshire.....
The roads are great between Dubois and Williamsport in Pennsylvania....(I'm out of Toronto, so we pop down to Pennsylvania for our 6 day rides)
No biker should miss the tail of the dragon (google it hwy 129 211 curves in 11 miles)- do not ride it on the weekend - too busy.
The blue ridge parkway is breathtaking (again, can be pretty slow on the weekends, it begs for 40-50mph but it's 25mph in places and it's a national park - don't get caught speeding), but cris-crossing the blueridge all the way down to the dragon is a great trip....
We used to stop at a different motel every night, but we've come to appreciate riding to a spot and staying for 4-5 nights and just doing day trips without the luggage...and we stay in places with efficency kitchens and eat in the room, a little cheaper, and after a days ride, we enjoy a good shower, and the peace of eating in the room with the AC blasting rather than a restaurant and all the noise.
pull up google maps, look where the parks are, look at elevation changes (makes for twisty roads).....
keep the rubber side down!