Absent a freak weather event, Labor Day weekend would still be warm at any of the locations listed.

My personal favorite is the Outer Banks of North Carolina. Beautiful beaches, decent restaurants, and cool historical stuff (like the Wright Brothers Museum). (The only thing it really doesn't have much of is night life. If you're looking for that, your best bet is South Beach.) If you go to the Outer Banks, a good place to stay would be the Sanderling Inn in the town of Duck: http://www.thesanderling.com/

The one potential downside at this time of year (as with most of the beach locations noted) would be the ultimate weather event: Hurricanes.