I second (third, fourth..) the recommendations for Acadia. I love staying near Southwest Harbor, especially camping at the Seawall campground. Bliss!
This may sound like an oddball suggestion, but a few people mentioned the Fife Brook section of the Deerfield. Zoar Outdoor has a fantastic paddle school that includes whitewater canoe instruction. I love whitewater paddling and have been doing it for years. But: I HATE, hate, hate rafting on whitewater. I get more bangs and bruises in rafts than I ever do in my solo boat, and you don't really get the river feel at all--just the frustration of trying to muscle an unweildy rubber beast, and if you flip it's a pain in the ***.
I love solo whitewater canoeing and highly recommend taking a lesson via Zoar if it's something that really interests you. Canoes take a bit more skill, and the learning curve is steeper at first than for a kayak. Which also means more time on easier whitewater, building skills. But in a canoe, you're kneeling fairly high up, so you can see/read the river better than kayakers, and also there is no spray skirt, so you don't get that feeling of being "trapped" in the boat if you flip. If you flip a canoe, it's nbd, you just fall out, no action required on your part. I found that when I switched from ww kayak to canoe, I was much more aggressive, because I wan't afraid of flipping and having to execute a roll, which never was my strong suit.