The aquarium is pretty good, Bull and Finch pub (the outside is from the start of Cheers), Pub inside is a bit rubbish though. Go on a tram / bus tour round Harvard and Cambridge.
Take a walk down Commonwealth Avenue reputed to have been Winston Churchills' favourite street.
3 days is probably enough time to see things there, make sure you take your passport with you when you want to go for an alcoholic drink!
Thanks for all your suggestions.
Did - Fenway Park, New England Aquarium, Freedom Trail. Prudential Sky Walk, Quincy Market, Faneuil Hall, Star Wars exhibition at the Science Museum, Duck Tour and still had money to buy perfume at the duty free!