I live in Maine and have three older kids, and we have almost always rented a house by a lake for a week each year through Airbnb or VRBO, and it’s fabulous. We swim, paddle, etc.
We also stay in the White Mountains in NH regularly and enjoy the mountain hiking, lakes, rivers, waterfalls etc.
Maine has beautiful beaches, and interesting towns - Portland ME is one of America’s best kept secrets, and we have lots of cool towns like Bar Harbor, Boothbay Harbor, Bath, Newcastle, Portsmouth NH, etc.
Most of it is uncrowded and not crazy expensive. The house rental will set you back a fair bit mind you.
Acadia National Park is gorgeous but crazy busy in summer. I’d avoid it tbh - same goes for Cape Cod.
Boston is a 2-hour drive from Portland. It’s fun for a few days with teens.