Just adding to the chorus of voices urging you to visit Fallingwater. I visited Frank Lloyd Wright's masterpiece in 2000, during a visit to Pittsburgh. I was fortunate enough to get a ride from a fellow fan (and you will have to drive, there is no public transportation to this rural area).
Being from Chicago, I was of course aware of Wright's style. But I had no idea this house existed. The care Wright took to build a structure that would blend with, rather than disrupt, the nature surrounding it, is truly awe-inspiring. I briefly studied architecture in college (not to be an architect, more from an artistic perspective), and I think anyone interested in architecture or taking care of the environment should make a point of visiting Fallingwater.
Visit the official site before visiting, it's full of practical tips.
On a bizarre note, the environmentally-sensitive public toilets are a sight to see in and of themselves.
On the way back to Pittsburgh, stop at one of many stands and get some homemade apple cider. :-)
Favorite thing: This is a Frank L. Wright masterpiece. To get there travel east from Pittsburgh on Interstate 70 to Jones Mills then south on highway 381 , to Mill Run . They give very interesting tours and it is best to be there before 10:00 am to aviod crowds. On hot humid days if can be like an oven in the house. The dense forest traps the humidity and the creek just makes matters worse. Gigantic Rhodedendroms grow in the area. Well they are bigger than the ones that grow in Canada .