Walden Pond, Boston
Technically Walden Pond isn't in Boston but in Concord, MA. But it's only a short hop away and I think you would want to see it, if only because it was associated with Thoreau (I assume you've heard of him :) Walden Pond is also a scenic and peaceful area perfect for contemplation and inspiration.
Thanks to Thoreau's writings, Walden Pond became packed with tourists in the late 1800s and early 1900s. Thankfully the throngs of people are no longer there. Though the pond is still popular for recreation, efforts have been made to limit development and keep it as close to its natural state as possible. A hiking trail circumnavigates the pond and offers nice views of its crystal clear water.