St. Augustine has a lot of bang for the buck, there's the old fort which is a national park, the old town (photos here of an old drugstore, hospital, cemetery, and the two oldest houses in St. Augustine)
There's also a beautiful bay, marina, lots of sailboats ( you can see the lighthouse in the photo across the bay from the fort above)
The St. Augustine lighthouse is beautifully refurbished, including the keepers cottage that I have a photo of the back of here. If you visit St. Augustine the lighthouse and the old fort are the two must go places.