Visit Fort Matanzas National Monument

Coastal Florida was a major field of conflict as European nations fought for control in the New World. As part of this struggle, Fort Matanzas guarded St. Augustine’s southern river approach. The colonial wars are over, but the monument is still protecting—not just the historic fort, but also the wild barrier island and the plants and animals who survive there amidst a sea of modern development

This gem is a great little excursion from St. Augustine. Take the free 5-minute ferry over to the old watchtower that guarded the city's southern approaches. It's in pretty good shape with good views of the inlet. Free.