Don't underestimate the power of a deck of cards or a board game. Simple card games -- whether they use standard decks of cards or are specialized card games like Uno or Apples to Apples -- can keep children occupied for a long time.
Several board games come in magnetic versions specifically designed for travel. The board is usually a metal case, and the pieces contain magnets that stick to the board. That way your children can play their favorite games and you don't have to worry about losing the pieces.
A different type of contained game is the "wipe off" or "wipe clean" game. These games are played on dry erase boards that can easily be wiped off and reused. Dry erase boards also give your children a place where they can draw without making a mess and scattering paper all over the car.