Falling Water is cool. I saw it about 15 years ago – before the shut it down and beefed it up, structurally. It’s a long drive from Philly, though… we spent the night in Pittsburgh when we went.
If you’re only interested in modern homes, there’s a Philly realtor, Craig Wakefield, who has a website about Philly modern homes. http://www.modernhomesphiladelphia.com/
I geared my reply towards Philly since I know you spend time there and Falling Water is in the same state (although other end.)
Locally there’s the Marston House, Whaley house, etc.