We had one full day in Portland, primarily because after we checked out of Chebeague, and took the ferry back to the mainland, we had a 6pm reservation at Fore Street Restaurant. It's so good, it's reason enough to go to Portland. I think it's like the Zuni of the East Coast. If you're going to be in the area, call and make a booking, even if it isn't the day or time you want--and then several days before they often call you up and can give you the timing you need.
Mackworth Island, where you can see fairy houses, and make your own too.