Here are a couple to get you started:
http://ridewithgps.com/routes/810965 - Starts in historic Lexington Center (of shot heard round the world fame), you can get to the start on the Minuteman Bikeway from Cambridge.
http://ridewithgps.com/routes/111317 - Starts in historic Concord Center (of North Bridge battlegreen fame), one major climb but well worth the scenery and sense of history.