Another option that would include both some history and architecture along with a scenic drive would be, as you note, a drive down to Washington and back which would include roads and places like:

US-219 from Buffalo down through the Allegheny National Forest to Pittsburgh and the Allegheny County Courthouse & Jail and other architectural sites, then taking the Turnpike, PA-31/PA-381 to Falling Water, the Frank Lloyd Wright masterpiece near Ohiopyle, PA. From there, it's a relatively easy matter to continue south to US-40, the old National Road past Fort Necessity and Fort Frederick into the Washington, DC area and the National Mall and government buildings. A side trip via US-50 to Annapolis would also be in order here.

Next you could head north on US-15 to Gettysburg and up the Susquehanna Valley to Watkins Glenn and then County Roads 23/114/96/87 to NY-54 and I-86/I-390 to Letchworth State Park and return to Buffalo on NY-36/NY-63/I-90 with a final visit to another Frank Lloyd Wright designed building, the Darwin Martin House.

Additionally, you should be aware that most car rental companies, while they will rent to someone who is only 21, usually charge a substantial 'underage driver fee', and you should make sure that you are clear on what that fee will be before you show up to get the car.

AZBuck