I've been fishing that river since I was 3 or 4 years old, I'd definitely consider it one of my "home" rivers.
There is a 17 or 18 mile trail leading from the upper campground to the lower. You need to find out how much rain they've had because it doesn't take much for the Cranberry to get muddy, flowing fast, and possibly flooding the lower campground.
Bikes are an absolute must if you plan on fishing all the good runs. There are a couple ways to go about fishing the Cranberry:
1). Go with a friend, and leave their car at the bottom campground. Drive to the upper campground, and bike down the mountain. If you want to stay in the back country, bring one of those carriages for your bike so you don't have a lot of weight, it doesn't take much to be worn out from exhaustion. Stay however long you planned, return to the car at the bottom, pack up, and drive to get your friend's car.
2). Camp in the lower, and bike as far as you can handle, and fish back down the mountain. The river runs along the trail for most of it's length, and if it isn't immediately accessible, it's a short walk through the woods.
3). Stay at a bed and breakfast in Richwood, and fish the lower Cranberry that runs along the road before you get to the lower campground.
Any way you go it is good fishing. I've never fly fished it as I've only been fly fishing since I made this account, and haven't been back, so I can't give you any fly selections. I'd assume a poofy dry like an EHC or stimulator would do well, and a nymph dropper where it's deep enough would be a good start.
Keep in mind that I am 19, so be weary of fatigue. Dehydration needs to be considered because of how humid it is. It's so humid that in the mornings it feels like you take a drink with each breath, but you'll sweat like you have never sweated before. Also pack a lunch, it helps keep you fueled, obviously.
Have fun, let me know what flies work for you!