Ideally, one would be travelling in a car such as this:
photo by wackystuff
Or perhaps this:
photo by kat
But any vehicle will do, just as long as it can take you from A to B to C and back again.
As far as i'm concerned, the two most important things to have before beginning any sort of road trip (apart from the vehicle) are as follows:
1) sunnies and a headscarf
2) homemade mix cds, painstakingly compiled to suit every route you'll be taking and stop you'll be making!
Here are some more roadtrip supplies from etsy:
light reading for the journey from VintageMaryponyup
road trip album kit by sweettweelab
vintage thermos set from RiverBazaar
what do you pack in your suitcase when you go on a road trip?