On that budget you might want to consider the Florida Keys, which could be the cheapest option for you from Pennsylvania.
Hawaii might be out of the question in terms of airfare, but once you're there it is very easy to sleep out on the beaches. I spent a couple weeks on Kauai once just tenting on the beaches. I could have gotten a fine, but never had any problem, and lots of locals were doing it (although they could do it legally apparently).
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