I'd concentrate more on South in March than North, unless you want to increase your chance of seeing snow. I'd skip Cape Cod (Provincetown) and the Islands (MV) as they are very quiet in offseason with many shops and restaurants closed, although weekends are a bit better for options.

For 7 days, if 3 are in Boston, then I'd add no more than 2 other locations to the plan, and of the places listed I'd pick from Newport and Mystic and I'd add New Haven (Yale, great food) to the possibilities
For 10 days, if 3 are Boston, I'd add either the Berkshires or the Southeast Maine options to the list.

Really, I'd suggest this trip in late April into May to open up many more snowless options, but you are definitely still in snow season in March.

Which is why we moved from Boston after 50 years to California, too much darn snow for too long each winter.