The Boston Museum of Fine Arts is a very nice art museum, very well-respected and plenty large to fill a day or maybe more. The collection is pretty diverse. I found the gallery of New England impressionists quite interesting when I was there a couple years ago, partly because I had been pretty much ignorant of impressionism in America being much of a thing at all.
The Isabella Stewart Gardner museum, quite close by, is another entrancing art museum.
I assume you're probably already planning on this, but I would strongly suggest avoiding driving in Boston. Take public transit in and use transit or walking to get around. It is not a particularly easy city to navigate by car, and parking is not inexpensive.