You maps aren't to scale so you kind of have to imagine the overlaps here. You can see Lechmere at the end of the Green Line which is where the Cambridgeside Galleria and hotels are located.
Follow that line back down and you'll see Copley right before it splits into 4 branches. (Another reason why Copley in Back Bay is so popular? A funnel-in point for everyone a little further out?) Arlington and Boylston are also Back Bay.
If someone suggests "Waterfront" note that there are TWO: South Boston WF and Downtown WF. The Aquarium is on the Downtown WF. The Convention Center is on the South Boston WF. They are only 10-15 minutes apart but a mistake can throw off a schedule.
I think that Fenway, South End and South Boston are too far out of the way for many.
See the Red Line and find: Kendall Square, Central Square and Harvard Square in Cambridge. They are all good, except for parking. Porter, Davis and Alewife are a little too far out. (Major exception to us locals being BAD which recently relocated anyway.)
If you line up an accommodation, ask them their "neighborhood" and then you can post "I found a major hotel in Historic Downtown, how's that?" or "I found a boutique hotel in the Financial District, what do you think?"
Bring your winter woolies