Activities for those long car trips

We enjoy taking family trips, but nobody enjoys that constant "How much further" or "Are we almost there?" coming from the backseat. So my wife and I try a game we'd heard about to put a stop to the constant questioning. 

At the start of the trip we give each of the kids a zip-lock bag with some coins. If either of the kids wanted an "update" or wanted to know how much further, they'd have to spend one of their coins. 

You wouldn't believe how fast it stopped the questions coming from the backseat. For younger children you could put candy in the bag instead of money. We did give the kids regular, hourly updates (for free) to keep them informed on where we were in the trip.