I think Topsail SP IS under-rated, even by most locals. I'm an avid outdoorsman, and it wasn't until two weeks ago that I explored Topsail, though it had been on my to-explore list since I moved here several years ago. On the trail south of Lake Morris, I didn't see any other recent foot prints other than my own.
From Ray's link, above:
Topsail Hill State Preserve, FL
The "wow" factor: The sugary white sand and the vast dunes are just the icing on the cake, which is the almost-deserted, three-mile stretch of beach along the Gulf of Mexico. Beyond the beautiful beach, this northwest Florida treasure offers boardwalks over the protected dunes, cute bungalows for rent on a weekly basis, miles of hike and bike paths that are also open to roller-bladers and fishing. Ranges are plentiful and offer various classes that are both educational and fun — like how to fish in the surf.
Consider in place of: Nearby Destin, Fort Walton Beach and Pensacola — all wonderful beaches in their own right but way crowded during the high season. Topsail's beach is rarely crowded anytime of year.
Insider tip: If you're seeking pure isolation, visit in October when the weather and the water are still warm and the crowds are nonexistent.