I love Great Falls - its one of my favorite places to go. I think the Maryland side is the best for hiking- have you been to the Virginia side?
The Billygoat Trail starts at the Old Angler's Inn in Maryland, a little ways before you get to the main gate for the park. Its where all the kayakers go to launch. If you head up the tow path and get to the little lake, its on your left - there should be a sign. It has some of the best views of the Potomac and if the water isn't too high you can jump on some of the larger rocks and get to one of the big islands.
Damn, I really miss that place.....
D