I grew up in Carlisle, PA (near Harrisburg off of I-81) and unfortunately didn't start fly fishing until after I moved to TN. I have been able to get in some good fishing up there when I go back to visit and I must say the Yellow Breeches is my favorite stream to fish. A quick search on current fishing conditions turned up this positive report:
there is also the very famous Letort Spring Run, which I've found to be a very difficult stream to fish...and there's the Susquehanna, which is a fantastic smallmouth river but appears to be a little too high to wade right now...
Hopefully this helps!