The biggest fort in the area is obviously Fort Donelson, a National Battlefield Park near Dover, worth visiting, if not exactly UE:
http://www.nps.gov/fodo/index.htm A few other Civil War fortifications line the route from (Old) Johnsonville to Nashville, think there is one in Dickson, though most are simple earthen squares, none comparable to the brick Third System stuff along the coasts.
If you're unimpressed by abandoned houses, the furnaces won't be much better as most are just stone ruins from the 1830s or so:
http://www.landbet...ed-activites/iron/ Worth checking out
http://en.wikipedi.../wiki/Mound_BottomTechnically protected, but never saw anyone there. Might need a raft to cross the river. The Montgomery Bell Tunnel, about a mile or two down, is more accessible. Short tunnel, historical and photogenic, otherwise not much to explore.
The E.T. Wickham statues near Palmyra are also worth seeing, folk art scattered throughout a large property, many heavily vandalized by local kids, gives a creepy vibe.
http://www.roadsideamerica.com/tip/6757 Unfortunately as you said, you are in the middle of nowhere. Never had an issue with locations in Nashville, though conversely I was always mortally freaked out by poking around the countryside and coming across meth heads/moonshiners/good ol' boys/neo-Confederates.
Then again, I'm not white.