This is the freedom trail site, that lists pretty much the "must see" sites in Boston.
http://www.nps.gov/bost/bost_lograph...l.htm#sitelist
(Quincy market and Fanueil hall I think are pretty overrated, the Old North Church (Paul Revere and his lanterns, Bunker Hill and the COnstitution are probably the best)
If you are a baseball fan, a swing by Fenway Park is always fun for people.