Not a strictly aviation point, but try to get 3rd party car cover before you go. American Express offer a policy that covers hire cars and (hopefully) the 'courtesy cars' that are still offered at many rural airports. The courtesy car is a huge convenience compared with renting but the snag is that the owner will often assure you that it's 'insured' not realising that because you are a foreigner your personal car insurance doesn't cover you in the US. Driving uninsured is illegal in many states and a seriously bad idea. Sorry if you knew this already.