If you drive from Washington DC to Niagara Falls, the most reasonable route is to travel northwest to Pitsburgh, then north to Erie, then northeast to Buffalo and Niagara Falls. This is somewhat indirect, but there are no direct routes via interstate highways. A direct route would require many miles of travel on rural roads through the Appalachian Mountains of Central Pennsylvania. It would probably involve more hours of driving than the interstate route.
If you go on the interstate routes, you can stop in Erie, Pennsylvania. This is a small city on the shores of Lake Erie. It has nice beaches and a small amusement park on the shores of the lake. I am sure you can find a variety of reasonably priced accommodations in Erie.
I am not familiar with the route from Niagara Falls to Boston, but between Albany New York and Springfield Massachusetts, you must pass through the Berkshire Mountains. This is a well known scenic area with many resorts. I believe you could find some accommodations west of Springfield.