Boston and New England is one of our favorite places in all the U.S. We live in the western U.S. so it's probably the charm of something being older an dhaving more history than just 1880 or so.
At any rate, all of the good places have been mentioned... The Freedom Trail is walkable and very interesting... Old North church, Paul Revere's home and more. The Trail is well marked. It's actually a 2 foot wide red brick inset in the sidewalk. (Seen here:
http://www.cityofboston.gov/freedomtrail/ )
As already metinoned... start your tour at Faneuil Hall... seen here:
http://www.faneuilhallmarketplace.com/
If you have time, it would be a great idea to pick up a rental car (ya'll call it car hire, no?) and take the easy, extremely invigorating drive up to Portland, Maine, about a 2 hour drive on Interstate 95, and just a few miles more on Interstate 295 to Freeport, Maine. It'll take 3 or 4 hours to tour L.L. Bean's home store. While there, be sure to eat lunch at Harraseeket Lunch & Lobster... try the lobster chowder (or as they call it
lobsta chowda in Down East Maine-speak).
If it's any date after September 15 or so, the colors (colours?) will have begun with the maple trees.
There are numerous Light Houses (or Lights, as they are called locally) to see and the rugged Maine coast is a rare pleasure. The most widely known is Portland Head Light (seen here:
http://lighthousegetaway.com/lights/maine2.htm l )
At any rate, welcome to the U.S. (You'll be well received in Boston if you have an Irish accent!)