Depends on when you go (and maybe who you're with)...
If you go in the spring, I'd try Williamsburg or OBX.
If you go in the heat of the summer, I'd definitely go to Cape Cod.
If you go in the fall, I'd be back at Williamsburg or OBX.
Williamsburg is a great destination because you can combine thrills (Busch Gardens) and history (Colonial Williamsburg and nearby Jamestown). All are wonderful places to visit, but I wouldn't really want to walk around any of them in the humid summers of Virginia.
Cape Cod is great, but since it's up in Massachusetts the ocean water won't be warm until well into the summer. The beaches are terrific, with great breezes to avoid feeling too hot.
Outer Banks is nice too, and since it's much further south, you'll be able to enjoy the beach in the spring and fall.
I vote for spring break, go to the Outer Banks. During the summer time, Williamsburg is awesome. You can go to Busch Gardens and have a blast there - bring a picnic lunch with you, the food is expensive and not so good. Cape Cod is cool, but also humid in the summertime. It depends on what you prefer ...
OBX (:
