What is an absolute must see?  I will be visitng for the 1st time & want to make sure we do not miss any of the action.

Parts of Cambridge perhaps too.  Harvard Square?

The North End is a great place to wander around - it's a neighborhood kind of place - lots of little shops, windy streets, and, if you want a piece of the World's Best Pizza, the original Regina's Pizzaria. Boston is actually a really foot-accessible palce, with the North End and Fanueil Hall adjacent to one another. Have fun!

there are a lot of cool things in Boston...
Walking around the back bay area can be interesting, out to eat at various pubs in the Northend is always a good time too.
The museaum of fine arts, the boston aquarium, the Harvard Botanical museam....all uniquly Boston....
However, the one thing that is so Boston as to be a MUST-DO:
Go to Fenway Park and see a RedSox Game. Like baseball or not this is a piece of history and one of the last remaining Classic ballparks in the country.
That is hallowed ground here in the Northeast but we are willing to share-

They take you on a tour of historic Boston and then into the Charles River. If your really good they let you drove the boat.
Seriously, I did it with a friend last summer and it was a real hoot!

AR
XO

a great selection of Italian restaurants, Harvard Square for the atmosphere even with school out, the Back Bay, there's lots to see & do.  Two unique restaurants right down by Quincy Market, Durgin Park and Union Oyster House, both uniquely Boston.  And all of the Freedom Trail.  And just about everything else the other guys mentioned.  And as somebody said, it's a great town to see on foot (and an awful place to drive!).

I miss read "Duck" and thought there might be a great new tour.