If you're looking for a historical tour of Boston, i would recommend the duck tours. It's an amphibious vehicle that goes around the city then it plunges into the Charles river. If you want to walk, you can do the freedom trail, which starts in boston common and ends up in Somerville. It's pretty long, but i feel that boston is a walking city. But it really all depends on the weather. The boston aquarium is also really interesting. Try to go in the morning of a weekday. If it gets too crowded, you can't really enjoy all of the exhibits.
If you want to shop, check out copley and prudential mall and Newbury street. Harvard square is also really cool. That whole area is i interesting to just walk around. Plus you can walk through Harvard yard which is nice.
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