One of the strange things about Boston is in the middle of the concrete jungle there can be a historic site, church, etc. Lots of that stuff was built around, so don't let the overhead view fool you.
If the weather is good, strolling along the Charles is also a nice way to spend some time, watching the boats go by.
This site
http://www.trails.com/activity.aspx?area=15130
has many suggested walking tours for the Boston area (and many other parts of New England, probably the entire country, never used it outside New England)