Welcome to the RTA Forum!

In answer to the easy question, renting a car will be your best option. You'll have to do a little shopping around to see where you can get the best rate though. Buying a car would be impractical and too expensive for a 2 week trip.

The price of gas in the US is about $2/gallon right now.

Otherwise, the US is such a big place, you really do need to just start doing some research about the kinds of places you want to see and things you'd like to do. Once you get an idea of the basics of your trip, we'll be in a better position to help you find some of the other things to help fill in the details.

You should also know that Route 66 was decommissioned 2 decades ago, and while there are sections that still exist, its a historic route. You should also know that drive in theatres are few and far between, with no more than a couple hundred left in the entire US.