When I still lived in the Boston area I used to regularly visit the Mt. Auburn Cemetary in Cambridge, mostly because it's one of the most beautiful places I know of. There's amazing history there, too, as a lot of prominent names are buried there, and the varied sculptural styles of the stones from various eras is fascinating...there's also a tower set on a hill which you can walk up inside and get a wonderful panoramic view of the surrounding area, including Watertown, Belmont, Arlington, parts of Boston. Simply one of my favorite public spaces.