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Well, since I used to live in upstate New York and had sisters in both Washington and Boston, I've done both legs of your proposed journey and can tell you that you're in for a treat. But three days may be pushing it just a bit if you really want to stop, see, and enjoy everything along the way, So let me lay out some of your options and then you choose which you'd like.
From Washington, leave on I-270 to Frederick, MD where you'll take US-15 north to Gettysburg. If you had some more time, you could take a short side trip from Frederick to Antietam, site of one of the bloodiest day's fighting ever.
Amish country is a bit off your route, but if you want to experience it, take US-30 to Lancaster and then get off on some of the smaller state highways southeast of town such as PA-340, PA-372 and PA-896. The Amish are intensely private people with a strong sense of community. I would ask that you respect their privacy and simply enjoy the drive through the lovely countryside that they maintain. Try to be somewhat surreptitious in your picture taking and keep your speed down - you never know when you'll meet a one-horse buggy in the road. You can tell the Amish farms from how well kept they are and the fact that there are no power lines connected to them. Anything advertised as an "Authentic Amish Experience - Come Visit" almost certainly isn't, don't.
In any event, from either Gettysburg (via US-15) or Lancaster (via PA-283) head up to Harrisburg and use US-15 north along the Susquehanna River. Around Williamsport you'll leave the river valley and head through some low mountains and forests, but this entire drive should be quite nice. In southern New York, another short detour up NY-414 from Corning will bring you to Watkins Glen State Park, one of my favorite hikes in the state. But now it's time to boogie a bit, so take I-86 (NY-17) and I-380 northwest towards Rochester with another great possibility for a stop at Letchworth State Park before using US-20A into Buffalo on the way up to Niagara. If your rental car contract allows it (read it carefully or ask at the counter when you pick it up) cross over to the Canadian side as well for a different and complete view of the falls.
Hmmm... You see what I mean by saying that there is way too much to see on this 'dogleg' trip to fit into three days. So far I've only covered half of it. Rather than bloviate any further, let me direct you to this post which lays out the basic scenic trip between Niagara and Boston. Note that in that post I even questioned whether three days was enough for that leg. Ah well, better too many choices than not enough.
AZBuck