There's actually a lot to see in between Boston and Washington. It's a modest drive in one day (440 miles or so), very easy in two.
How about heading south and looking at the lovely mansions and cliffs at Newport, R.I. (mid-morning of first day), then take in the harbor at Mystic, CT in the afternoon, then retire for the night in Bridgeport or Stamford.
On the second day, visit Lancaster, PA and see Amish country before dropping down into Baltimore and Washington.
Not a lot of time for extensive sightseeing, but this routing should be fairly easy and allow you to see some history and scenic beauty.