I do love visiting cities that soak me in history, and Boston is a great destination that can be a great travel partner for any solo traveler. The city itself is very walker-friendly, so let's go!
First off, get a T-Pass - The T, also the Massachusetts Bay Transit Authority, is the best way to plan to get around. If you're not comfortable with traffic or aggressive drivers, I don't recommend renting a car in Boston. The T makes it easy and convenient to get around. Look for a Charlie Card and Ticket vending machine. You can buy a 7-day pass for a bargain in comparison with cab fares and car rentals. Check out their website for info.
T-Pass in hand, you're ready to see Boston. Put your best walking shoes on! You're going to log some miles.
You'll take the Blue Line from the airport, and presuming that you're settled in at your hotel, head for Government Center. It's there that you're within a short walking distance of Boston's The New England Aquarium.
The New England Aquarium is one of my favorite things about Boston. On the harbor, visitors are greeted by the Penguin exhibit, and these classic characters of every zoo can make anyone smile with their antics. Walk up the ramp and marvel at the sea turtles, sharks and other marine life in the center column.
Once you've soaked up the bliss of the Aquarium, you can hit the Freedom Trail in a few hundred steps, and make your way to the USS Constitution, the oldest commissioned warship in the world. With your US Navy tour guides, you'll learn a great deal about what it was like to live and fight on this grand ship in the earliest days of our country.
From The Constitution, you can follow Freedom Trail to Paul Revere's House and the Old North Church, and then make your way back to Faneuil Hall and Government Center. From there, catch the T back to Copley Square, where you can see the finish line for the Boston Marathon and also marvel at the architecture of The Boston Library. While there, you're in striking distance of the Museum of Fine Arts - which could take an hour or a day of your time, depending on your love of art.
One perfect weekend in Boston can be had - Faneuil Hall, the New England Aquarium, the Freedom Trail and the sites it marks, Copley Square, and the Museum of Fine Arts. That's my recommendation. Oh - and a Duck Tour!