Lots of good comments already, that I pretty much agree with.
Acadia (which has no "r" in its name) is certainly worth the visit. Coastal Maine in general is lovely.
Boston is an interesting city in terms of history and culture. I would strongly suggest not attempting to drive in the city at all. As chance would have it, just yesterday I was in Boston to see an exhibit at the BSA Space (on unique playgrounds and playspaces) and to generally enjoy the day and the ambiance. I took the T from Alewife.
If you do decide to visit Niagara Falls (which, as was said, is not at all near the coast), do consider spending time in the finger lakes and Adirondiacks of New York. There are some spectacular state parks especially in the finger lakes region, such as Letchworth and Watkins Glen. They don't offer "class A" campground amenities (not even full hookups), but the parks are absolutely beautiful and unique.