1. Slug Bug: This game can be played without the slugging, mind you. The objective is to accumulate points over a specified amount of time by sighting a specified object, normally a certain type of car. In the example they used VW Beetles (Bugs) and PT Cruisers (Cruisers). I think that you can make up your own version and set the point values accordingly.
2. License Plate Games: There are several versions of this game, the most common is counting license plates from certain states or who can see the most plates from different states. You can use a checklist system to check off the ones you have spotted or you can go through the alphabet. Once you have seen a state that starts with A you move on to the next letter. Another fun version is to make up words with the letters from the license. The words have to use the letters or must be in the order of the letters on the plate, such as BUS could be business, bust, bustling, busy, etc.
3. Scavenger hunt: First, you have to come up with a list of items that you expect to see along the way. Then there are points assigned for each item spotted, first one to see it gets the highest value awarded.
4. Cow Poker: This one is played in teams one either side of the car. Each sighting is the total of cows spotted. There can be things that will cancel out the cow spotting, like a cemetery or a butcher shop, and some random extra points for lone chimneys, a black horse, ice cream truck and the like. These things and points should be decided before the game starts.
When my folks and I go on a road trip, mom and I spot license plates and then try to come up with the state bird or flower. After our long drive back from GA a couple years ago, we had gotten pretty good at those so now have to find another state related item to learn.