Who can locate the most animals during the trip? Any creature that moves (or once moved) can be fair game, so to speak: deer, livestock, birds (get them to name specific types), road kill and even sun-dried insects caught between the dashboard and windshield can be counted. Make this as competitive or non-competitive as the situation warrants -- perhaps put up a small prize to encourage everyone to participate. You might even want to appoint someone to write down who said what, just so you know who gets the prize at the next rest stop.
But even imaginative kids can get bored of mind games after awhile. On the next page, find out how to offer some inexpensive incentives to keep them playing.