Alafia River State Park has some decent hiking trails, as does Myakka River SP. Up north, the Stephen Foster Folk Center has some cool trails, including a fairly long trail along the Suwannee River. All have electric and water, and Myakka has full hookups in the Palmetto Ridge section. Further south, we like the Midway Campground on the Tamiami Trail in the Big Cypress National Preserve section of the Everglades. Sites are electric only, but there's water available near the dump station. There's some pretty good hiking trails nearby.
