Be sure you dress comfortably. You don't want tight clothes or clothes that chafe. Take your shoes off, and get comfy! :).
Pack a small grab bag. Bring a camera. You never know what you'll see. Have healthy snacks and drinks available. If you aren't prone to car sickness, you can read or download some apps to play. Be sure to charge the batteries in your phone, tablet, music player... Talk with your family! Listen to music. Stretch during breaks!
Be safe! Wear your seatbelt and try not to distract the driver.