I agree with everything above.
The Public Garden and the swan boats, especially if you ever read "Make Way For Ducklings" when you were a wee bairn.
I can't remember if the JFK Presidential Library is free/cheap.
Check out the schedule of speakers at the Harvard bookstore to see if anyone interesting will be speaking during your visit.
For the niche market of "Car Talk" fans: gaze up at the office window of Dewey, Cheatham and Howe in Harvard Square, and then make a pilgrimage to the Good Times Garage in Cambridge.