A couple ballast resistors, a spare ignition box, oil. And food and water. That way if you're stuck along side the highway for any length of time, you've got snacks. (And if you have your carry permit, and if it's recognized by other states, a pistol or three)
I know you're wanting to go down I65 but to me, that's not a true road trip. Speeds too fast to really see America. Why not take US 31 , runs nearly the same path as I65 and ends in the Gulf area as well? That's where you'll find epic views and sights more so than taking 65. My wife and I took 65 all the way down to the Gulf in 2015. Was pretty boring until we got off of it in Alabama to make our way to Destin.