Favorite thing: Don't miss Fallingwater, the historic home designed by Frank Lloyd Wright. It has a stunning natural setting over a creek--only you can't actually see the creek from the house (you can hear it, though). The house is so well integrated into its surroundings it feels a bit like a treehouse. Not all the doorways and windows seem practical, however. Then you can go see the mighty Youghiogheny (pronounced Yock-uh-GAY-nee, or simply 'Yock'), which the daring will want to whitewater raft. Go for it!
Take a river Trip! At least 7 miles of class III and IV rapids, excellent shuttle service and plenty of ways to enjoy it. Novices can take a fun guided raft trip, or experienced kayakers can hit the rapids in a kayak. See: http://www.dcnr.state.pa.us/stateparks/parks/ohio.htm
Fondest memory: I have paddled it at all times of the year, and my personal favorite is in the fall when the river is less crowded. Here I am surfing it up in November...