The Freedom Trail is a three-mile route connecting 16 points of historical interest marked by a row of red bricks or a painted red line on the sidewalk. After Quincy Market it's the most visited attraction in the city. Unfortunately it's not a loop, so when you get to the end you're a couple of miles from where you started!
The walk is easy. It's up to you how fast or how slow to go. Some do it in a half-day, walking past sites without more than a glance while others feel compelled to go inside every building on the route and take two days. However, not all sites are created equal. Plan on the better part of a day. Take your time. See what interests you and have lunch somewhere along the route but get a guide book or free brochure (from the National Parks Service, corner of State and Devonshire, next to the Old State House) so you have some idea of what you're looking at!
Sites are in no particular order - they're a jumble covering buildings and events from different times - so your route doesn't matter. Start at the "end" of the trail, at Old Ironsides in the Charlestown Navy Yard. Go on the ship and take a look in the museum - it's free. This end gets crowded as the day goes on, so that's another reason to start here. From downtown, take the water shuttle from Long Wharf, next to the aquarium. It's a great way to start your day of sightseeing - a 10 minute ride through the harbor to the Navy Yard for only $1.25.
After Old Ironsides, follow the red line up to Bunker Hill. Climb the tower if you must (free, but 294 steps). The view must have been better when it was first built, but it's not much these days. Then it's over the bridge to the North End and Old North Church, Copp's Hill Burial Ground and Paul Revere's House. If there's not much of a line, stop in at Old North - a speaker regularly gives a 10 minute summary of the church and its moment in history. Continue on, under the Expressway, to Quincy Market and Faneuil Hall. Find the entrance to Faneuil Hall (it's neither obvious nor inviting) and go up to the second floor. There National Parks Rangers describe the Hall and Boston's role in American revolutionary history in a 15 minute talk. The Hall is air conditioned and it's a great place to rest before moving on.
Grab a snack in Quincy Market, then follow the trail past the Boston Massacre site, Old State House, Old South Meeting House, Old City Hall (do you notice a theme here? This is the old part of town...). Next it's King's Chapel, a couple of burial grounds (gravestone rubbing no longer allowed), Park Street Church, the State House and Boston Common.
I'm tired already and I'm only typing...Enjoy your walk!