I agree with the prior poster...Outer Banks would be my choice...there's not a lot of campgrounds to choose from, but one of the best is the Hatteras Island KOA. It's an oceanfront campground, but there are dunes between the campground and the ocean to traverse...they have a beautiful walkway and deck to the ocean...and although the sites would be considered close together, I don't foresee a problem with it being crowded for the dates you said. From the Philly area you're looking at an 8 hour drive or about 375 miles...believe me the place is absolutely beautiful if you want a nice relaxing break. Here's a link for you to a take a virtual tour of the Hatteras Island KOA:http://www.koakampgrounds.com/where/nc/331...tualtours/3.htm
It's only about 25 miles South of the Nags Head area and if you're in the Nags Head area be sure to stop at Jockey Ridge State Park...absolutely beautiful...huge sand dunes...you'd think you were in the middle of a desert....and just north of Nags Head is Kitty Hawk where the Wright Bros. Memorial is...(you'll pass this on your way down it's right on Highway 12.
With that said...if you're looking for more of a resort type of area then I would agree with the other poster who suggested Myrtle Beach...from the Philly area it would take you about 10 hours or about 600 miles to get there.
Wherever you end up....hope you have fun!!!!!!!