First, in the interest of full disclosure, I'm a guide for Boston by Foot (BBF) but I don't consider it a conflict of interest to recommend the tours. BBF is a non-profit educational organization and all tours are given by volunteers. I became a guide after going on a few walks: I enjoyed them, wanted to learn more, and now I'm leading visitors around the city. Since tours are given by volunteers, doing one tour every week or two, they're not a chore or a job for us - we generally have as much fun leading a walk as our guests do!
The two walks I suggest are "Heart of the Freedom Trail" and "Victorian Back Bay." The Freedom Trail is a three mile route through the city, starting on Boston Common and ending across the river in Charlestown, connecting 16 points of historical interest. The Heart of the Freedom Trail tour visits seven of these sites and a few other architecturally significant spots in a compact 1/2 mile loop. Guides stop at and talk about Faneuil Hall, Quincy Market, Old State House, Old Corner Bookstore, Old South Meeting House, Old City Hall (do you sense a theme here?), Kings Chapel and City Hall among others. Although all the guides undergo identical training - a mini college course in the spring consisting of classroom lectures, discussion groups, field trips, four written assignments and a final exam - each brings their own interests and enthusiasm to the tour so exactly what gets covered changes from one tour to the next. One guide may stress architecture, another history.
The Victorian Back Bay tour is more consistent. Its themes are historical - filling in the Back Bay to produce this land - and architectural. It starts in Copley Square with H.H. Richardson's 1872 masterwork, Trinity Church, then discusses the 1976 Hancock Tower and the 1888 Boston Public Library before moving to the residential section of the Back Bay. Again, this walk is a loop covering about a half mile in 90 minutes.
BBF tours start at various locations around the city at various times. Some are given daily, others once a week and still others are specials, just given once a year. Check the BBF website or brochures around town for details. One comment I've heard from people is "We weren't sure where we were supposed to meet you." Although the starting points SEEM perfectly clear to us, visitors don't always know how to get there. If it's not perfectly clear to you, ask someone at your hotel or an information booth for directions.
Current price (2006): $10 Adult; $8 children 6-10.