some boston ideas:
-Visit Walden Pond (yes, that Walden Pond)
-go ice-skating on Frog Pond in Boston Commons
-walk the Freedom Trail & look at everything historical
-visit Harvard Yard & surrounding
-go up the Prudential tower for the view
-go to beaches (Constitution Beach or Castle Island if you like planes)
-take a boat ride in the harbor
-visit JFK library & museum
-tour the USS Constitution (well, it's sorta on the freedom trail)
-tour the state capitol
-eat in Chinatown (Taiwan Cafe is good)
-eat in North End (Italy-town)
-attend a concert (Symphony Hall/BSO, New England Conservatory, Harvard, MIT, etc)
-visit the aquarium
-visit Museum of Fine Art or science or Harvard's Peabody or Natural History or numerous Art museums
-go skiing somewhere
-Six Flags?