As a former Maine resident and as daughter, granddaughter, niece, cousin etc etc of many many Mainers, I cannot stress this enough:
Sebago Lake. Water so soft you'll feel like you've got new skin. Water so clear you can see to the bottom in 20 feet of water. If you can rent a cabin on or near the water, it will be so worth it. I spent every summer of my youth shuttling between my home in New Hampshire and my Nana's camp on Sebago. It is heaven on earth.
I also recommend York Beach for all the rigamarole that it is. Popham Beach in Phippsburg, ME is fantastic as wellI concur with the rafting-- but there's also great non-whitewater paddling as well. Bethel, Maine is absolutely lovely if you want to head north.
I also really enjoy TRUE downeast Maine, beyond Bar Harbor, where the tourist madness dies off and the deep intermingling of Acadian culture starts up. It's a very cool place up there
Below is a shot I took five or six years ago of Douglas Mtn over Sebago![]()