If you could get permission to hunt the Annie Lytle School, I think it would be awesome. Built in 1917 and closed in the 1960's. A google search said its protected and guarded, others say its not exactly safe. I found it by accident back in 2005 when I was driving up from Orlando. Better hit it quick, looks like a recent fire destroyed 90% of the building. Demolition is all but certain now.
Have you found any good places to detect at in Jacksonville. I mostly do beaches when we go to them. Usually just end up cleaning the beaches with the junk people hide in the sand.