on road trips, each of my kids have thier own "bag" ... they have thier dsi's, IPod's, Small travel blanket & pillow in case they need it, a journal for writing and some of their favorite car friendly snacks. When they were younger, we had some of those color wonder marker kits and some toys. We also have a dvd system in the van so they can each pick some vidoe's, but honestly we spend most of our car trips playing different type of triva games, they rarely breakout the travel bag or electronics.
the other thing they each get (and this would depend on age, as it could be a choking hazard for small kids) is a baggie of quarters. They each start the trip with $10.00 worth of quarters and anytime they whine/argue etc or don't stop after being told (general misbehavior) they have to give mom & dad a quarter. @ the end of the trip, the quarters they have left is what they get to spend @ the arcade (or add to thier souvineer money). we don't give them any other $ for the arcade.
99% of the time they both have about $9.00-9.50 in quarters to spend which is not bad for a 12 hour trip and $20.00 for a peaceful trip is money well spent to DH & I, LOL!