At my age (19), anywhere outside of the U.S. is terrific!
My personal favorite is Arizona colleges' Spring Break favorite, Rocky Point (Puerto Penasco), Sonora, Mexico. You have to drive there and can't fly (except maybe GA
after clearing customs at some other airport) but it is cheap and has great specials that week, like $20 unlimited drinks and things like that. Let's face it, being between 18 and 20 in the U.S. isn't that great as far as things to do. Too old for kid stuff, and not old enough to legally do the adult stuff that comes naturally... which is why Mexico has become such a popular spring break spot in the past ten years or so (over previous faves like Florida).