Fernandina Beach...old town just north a ways. Nice drive up, red brick fort at Fort Clinch SP there. Via the ferry, cool ride.
Singleton's at Mayport...Diners, Drive Inna and Dives place...emphasis on dives, great food. Safe Harbor seafood...pick yours out of the case for cooking or order off the menu.
Savannah is probably doable as a long day trip...enough to make you go back for sure. If you mountain bike...super trails at Kathryn Hanna city park/campground close by. Beaches there as well.
Good stuff...enjoy the area. :) :)