Sebago Lake is one of the largest in Maine, and depending on where you go, about an hour from Portland. We went for a drive last week and ended up on the shores there, where you can hang on the beach and take a dip. It has sort of a local summer invasion feel and was a bit too crowded for my taste, but I can imagine it'd be really pretty in the off season. You can also go camping in the state park and there are trails to hike too.
Much like some of our other driving adventures, we were ill-prepared for swimming or hiking, but took a stroll along the water. I think it's so cool how the trees grow right along the edge, roots half in the lake.