Perhaps one of the most beautiful areas to walk in the Boston area is the Mount Auburn Cemetery in Cambridge. Contemplate life amongst the gorgeous headstones and admire the natural beauty of the rare trees, ponds and flowers that dot the trails and paths you'll tread. Make sure to climb the stone tower located in the middle of it all, where you'll get a stunning 360 degree view of the city.
The Freedom Trail is a path of red-bricks (or red paint) that wends its way through the downtown area connecting many of the historic sights of the city. In addition, you get to see a cross-section of the downtown that highlights both the old and the new and they play together.
In the mood for people watching? Walk the span of Newbury Street into the Boston Common. Aside from the detours you'll likely take into some of Boston's finest stores, you'll also get a good opportunity to get see some of Boston's hipfolk strutting their stuff.
If you're feeling like you need a break from the bustle of the city, try the Charles River Walk that stretches through Watertown and Newton. Skirt the water on a small, wooded trail that will take you over man-made bridges and boardwalks and under grand weeping willows.
No list of Boston walks would be complete without mentioning the Charles River Esplanade. Begin off of Soldier's Field Road in Brighton and make your way down into the city along the paved path that you'll share with bikers, rollerbladers and other people making good use of their legs. There's plenty of space along this walk to throw down a blanket, take a nap, read, toss a frisbee, or just bask in the sunshine.