An option you could look into would be to rent a car for a specific area of interest...say in the northeast. Then take a bus to another city or area of interest....say in the southwest, and rent another car there. Now I have not taken a bus myself but it could save you a bit of money.............especially through the midwest where there is not as much to see or do. Take it from me I live there Plus you can sleep en route.
you can check fares here....and they are cheaper if you book 7days in advance.