Drive 6-7 hr from Boston to Fall and find a motel on the way. The next day, drive an hour or two to reach the Fall. Spent all rest of day around the Fall. I believe it is plenty time for you.
Drive to DC in one day, or at least 80% of the road. Spend 2-3 in DC. There are plenty of museums at Mall. Don't try to finish more than 2 in one day. plus Capital and White house.
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If you have a small kid, forget about my suggestion above. And I suggest you only visit either Fall or DC, instead of two.