I figure most of us enjoy a good vantage point to observe the world, or at least over Boston.
94 or 96 from Davis), the walk into the Middlesex Fells is quite pleasant. Through the nearby neighborhood, walk north to the Fells through; I recommend Ashcroft Road.
Closer to Boston we find Prospect Tower in Union Square, Somerville (87 from Lechmere & Davis). It is on the top of a rather steep hill to the north of the square. The view of Boston and Cambridge from the top is fabulous, but always wintry.
Community College Station. The 221-foot tall tower commemorates the Battle of Bunker Hill, and affords a good 294 step workout to get to the top. This effort is not wasted since it is the closest tower to the center of the City, which provides striking views of the City that you can't get further away.
Of course, I saved my favorite tower for last.
Mount Auburn Cemetery (71 or 73 from Harvard). The experience is nothing short of majestic. I find it best to walk aimlessly through the Cemetary. Walk somewhere you haven't walked before. Work your way to higher points, eventually you may find yourself at the foot of the tower. Proceed to the top, it is a good climb but not tiring.