I will be in Boston for 3 days. What are the best places to visit? Things to do? Places to eat?

You are fortunate you will be leaving Boston after 3 days but 3 days in Boston can be an eternity.

First of all, think of it as Boston/Cambridge, not only Boston. The 2 cities stare at each other across the Charles River; both are prosperous and the subway system connects them. Not knowing your preferences for tourist enjoyment I cannot tell you the "best" places to visit or the funnest. But I will tell you that Boston is the most European-like city in the USA: very walkable (with a good subway system if for some reason - like heading for a place a couple of miles away - you prefer not to walk). Like New York Boston has an uptown and a downtown and its skyscraper district follow from one to another, shaped like a hook. The skyscraper hook is surrounded by historic old residential districts, mostly of distinctive architectures, many of which are still gaslit. Right in the middle between uptown (also called Back Bay) and downtown are two large parks - the Public Garden and the Common, on which it is still legal to graze your cattle. It is of course full of history, and the Freedom Trail is a walking tour connecting many of the historic sites. I suggest you buy a good guidebook. As for dining, you will probably get a lot of answers telling you to go to the Italian North End and indeed there are many delicious places in that district. But in fact the very best restaurants for combination of ambiance, fantastic food at reasonable prices and incredibly good wine lists at low cost are the Portuguese (including the Cape Verdean). Most Americans think of Boston for its Irish, Yankee and, to a lesser extent, Italian heritage but in fact southeastern New England has the USA's largest Portuguese and Portuguese colonial populations. In Boston/Cambridge the Portuguese restaurants are concentrated in East Cambridge. The best in our judgement are The Sunset, Portugalia and Casa Portugal. Otherwise, there are two historic (almost 200 years old) restaurants near Fanueil Hall - the Union Oyster House and Durgin-Park. In our judgement the Union Oyster House is overpriced and disappointing, but Durgin-Park should not be missed. Basic American fare served communually (but with good sauces and seasoning for the food, unlike Pennsylvania and Ohio Amish places). If you like fresh beer there are two brew-pubs under the same management which have excellent food. One is named Boston Beer house but damend if I can remember the exact name of the other. One is across the street from Fenway Park baseball stadium (there is also a Fenway Park with trees and grass) and the other a 5 minute walk from North Station.