There are two national parks in St. Augustine - the Castillo de San Marcos and Fort Matanzas. The Castillo is definitely a "must see" as it is conveniently located in the Historic area of St. Augustine. Fort Matanzas is located approximately 15 minutes south of St. Augustine and is a much smaller fort accessible only by boat (free ride) to the site, but definitely of interest to those that love history and a free boat ride!
This is one of our family's favorite attractions. It's a short drive from The Sand Bucket at Seaside and one that we visit often. This is a "must see" if you have children.