As well as Quincy Market and the North End (must go to Mike's) you could go over to Old Ironsides the Constitution. There is the Museum of Science, Fenway Park tours and of course the Duck Tours. They are a lot of fun. Especially when you quack at people. Freedom Trail, there is also a night tour of haunted spots in the city, that is on a trolley. I forget the name, but will try to find it. Boston Common and Public Gardens, try a swan boat ride then go over and have a beer at CHEERS. (Bull and Finch pub right near the Public Garden). All very walkable or a short cab ride away.