On a day trip from Boston, we visited Concord to see Walden Pond - the inspiration to Henry David Thoreau's book. It was peaceful and beautiful and the water was surprisingly warm and clear. We took a two mile walk around the lake, while our brother-in-law (the most amazing tour guide ever) read the first paragraph of the book, Walden, and regaled us with facts and history about the site.
Things to note:
The park rangers are very intense - be careful about getting out of a car on the side of the road, and they may close the park if it is too crowded.
Definitely bring a swimsuit, book, and picnic to enjoy a lovely and peaceful day by the water.