St. George is about it. There are a couple of CGs farther east near Carrabelle but they want resort prices.
It's been said that boondocking is available under the bridge but last I saw it was more expensive than St. George SP.
But since you're doing it:
Drive down to the St. Marks lighthouse. There's no camping but it's worth the trip if you're an outdoorsy type. There's a CG at the bridge across the St. Marks River at Newport; I understand the price may not be justified. I haven't been there for several years. Supposedly there's a CG at a fish camp down at St. Marks. Definitely do the boat tour at Wakulla Springs SP. Have dinner at the Spring Creek Restaurant at Spring Creek. Toddle on down US-98 and stay at Panacea http://www.holidaycampground.com/more.htm
-jbh-