We were just in St. Augustine in late March of this year, there's TONS of things to see and do there! We decided to take the trolley each day (3 days) to avoid parking hassles in the historic district, well worth the expense. If you have the time plan a trip to St Augustine Beach for a day of relaxation, if you do, get there early though as parking is limited down there. There's an excellent seafood restaurant very near the beach area, Sunset Grille (recommended by one of our tour guides). You won't be disappointed dining there! :B
We stayed at Compass RV Park, outstanding park!! They lead you to your site and they have a shuttle from one of several trolley companies that stops most mornings (and it's free) and drops you back off in the afternoon. It's a wonderful city to visit, I'm sure we'll visit again.
Dan