We parked near the visitors center in the parking deck ($12 a day) which we thought was a little expensive. You do have the option of using metered parking if you plan to drive from attraction to attraction. If you drive your rv, there is free parking for oversized vehicles.
First we visited Castillo des Sans Marcos, a formidable fort on the harbor. In each section of the fort, there was usually a timeline and exhibits of when the fort changed hands between the Spanish, British and the US. There were also a variety of demonstrations. I enjoyed the one showing how musket balls were made.
We also visited the Cathedral Basilica of St. Augustine. It is a must see. The stained glass and paintings are beautiful.
There are many, many touristy type shops if you like that sort of thing, but among them, there were a few interesting ones such as the mustache store.
All in all, it was a good day!