Another option would be to take public transportation to Arlington and ride the Minuteman Bikeway to Bedford and back. This is a heavily used bike path, but during the week it would be fine. I know there are bike shops along the path in Lexington and Bedford that rent bikes. I think there is one in Arlington; you could check that out. This would allow you to stop and see some of the historic things in Lexington. It's a fairly short ride to Concord from there, if you were really adventureous (not on the path, though).

There is a commercial bike tour of Boston that leaves from the Common; they provide bikes. However, this is city riding. I am sure this is listed on the web somewhere, under tourism things. I see it advertised in the Boston Globe every week.