If your driving at night, activate stealth mode. You just turn off all the light on the inside and outside of the car and see how far you can go without crashing.
Fuck with other people on the highway i.e. play running man where everyone in the car pretends that theyre running and speed up and slow down next to cars you pass. Also make the blow your horn gesture to truck drivers and stare/wave at other cars.
Get drunk as fuck (except for the driver of course). Drinking in the car is always a great time. Just be sure to bring a piss jug. Seriously though six hours is not a very long drive, do you really need to be entertained?