For history lovers it doesn't get any better than a trip to St. Augustine.
I love Old Florida in every form from coastal creeks and rivers to historic towns and roadside points of interest. All these things come together in and around the nation's oldest city and you can spend anything from a day to a month exploring the area.
Me? I spent about a week taking in the sights around town camping at Anastasia State Park and generally making the place my home. For a few days anyway.
Crossing the Bridge of Lions (a sight in itself by the way) into downtown you can't help but notice the massive fort to your left.
Construction began on the Castillo de San Marcos in 1672 and is the oldest masonry fort in North America. The walls are constructed from coquina a type of limestone formed from shells and sand that creates a natural concrete. Coquina for the fort was mined at Anastasia State Park (it wasn't a state park then of course) and ferried over in square chunks to St. Augustine.
Inside the Castillo de San Marcos it's easy to get a little overwhelmed. Each room is a lesson. Don't be surprised if you just head inside and sit to take it all in.
I'm a fan of going up to the top of the fort and watching boats ad kayaks go by in the Matanzas River. Good stuff and I did this three times while I was in town.
Here are a couple of panoramas to give you a feel of what to expect. One is just inside the gate and the other is the view from the top.
Learn more about the Castillo de San Marcos National Monument here: http://www.nps.gov/casa/index.htm