Here are some games we play in the car, which should be workable for the young set:

Alphabet Game Categories A My Name Is: Another Alphabet Game Grandma Went To Market I Spy
Tell A Story By Committee Create An "opposites" Environment Read Aloud Counting The Family Favorite
Scavenger Hunt Cow Count      

Alphabet Game: 
Starting with "A", find each letter of the alphabet on road signs, license plates, etc. This can be a group effort or taking turns.

Categories: 
Take turns thinking of items in a specific category. For instance Animals -- each person names an animal. Other categories can be colors, breakfast cereals, board games, fruits & veggies, musical instruments..... A variation on the category game is that each item named has to start with the last letter of the previous item. If the category is Places and a  person says "New York" the next person has to think of a place that starts with K.  Probably too much for a 3 yr old to do?

A My Name Is: Another alphabet game. 
Taking turns, each person takes  the next
available letter of the alphabet to complete the following story: "A my name is ________ my spouse's name is ______ we come from ________ and we sell
________" where all the blanks are filled in with words that start with thecurrent letter.

Grandma went to the market: 
A bit more difficult alphabet game, because memory is involved. On your turn, you name something Grandma bought at the market which starts with the next letter of the alphabet. But *first* you name all the other items Grandma bought. That is, first person says "Grandma went to the market and she bought apples". Next person says "Grandma went to the market and she bought apples and borscht" And so on.

I Spy: 
One person finds an object (in or out of the car) and says "I spy something ______" and identifies the color the object is. The other players then try to guess what the object is.

Tell a Story by Committee: 
One person starts a story. A few minutes into the story (usually in mid-sentence) the story teller passes control of the story to another player. The stories can get pretty unusual!

Create an opposites environment: 
Pick a place, say a pool, to describe. Each person in turn thinks of antonyms and describes how those word apply to the place. So a pool could be deep at one end, shallow at the other. It could also be hot at the shallow end (because there are no umbrellas there) and cool at the deep end (because all the umbrellas are there). The shallow end may be noisy because there's a restaurant nearby and the deep end is quiet.

Read aloud: 
If anyone can read in the car (I can't, but my son can) have the reader read a chapter now and then from a favorite book. Then you can discuss the story after the chapter is read.

Counting: 
Pick an object you'll see many of, but not *too* many. And for a half hour (or however long the attention span will last) count how many of those objects you see. For instance, count tanker trucks or silos or blue cars or the letter N.

Then there's always the family favorite that goes like this:
"She's touching me!" "Well he's on my side of the seat" "Are we there yet?" "I have to GO!!!!!" "I'm hungry. I'm really really hungry" and so it goes............

I often play many of the games mentioned above. With the counting game,  we each pick a color of car and see who can get to 10 (or another amount) first.

By far, our favorite game is one that we bought, but I suppose it could be made by hand also. It's called "Are We There Yet?". It's just a box of about 250 cards that each lists something to look for along the road.  One might say "a blue 2-door car", "a flock of birds flying together", "a yield sign", or "a waterfall". There are also "shortcut" cards mixed in. Each player picks 5 cards. If you get a shortcut, you can use it to trade it and another difficult card for 2 other cards. As you find your items, you put them in your "found" pile and get a replacement.  At the end of the time period, the person with the most cards in their "found" pile wins. We play cooperatively -helping each other find their items. This game has kept us busy for hours and hours! 

To play it with small non readers, you could probably use teams -each with at least one reader.

I was thinking about how my great aunt Emily kept us kids busy as we traveled. She is the one did the ABC and gemverses. While going through my things I found this list. She would prepare us each a sheet for these items. As a family (about 60) we would go several hours to a hunting camp in Northwestern Penna. for a week of fun. So she had to do maybe 20 of these. Those that came from out of state she would send theirs to them. We would all sit down and compare and she had prizes for all of us.  (tops, small books, magnant dogs she would get in a Bible Bookstore, raisins, and jump ropes and jacks. 
Scavenger Hunt 
This is a good game for long drives. Give child/children lists of things to watch for during the drive. Children can cooperate or compete to see who can find all the items on the lists.  The lists can be made up ahead of time or during the drive, and adjusted for the type of scenery and the age of the children.  Scavenger Hunt Examples: 

CITY LIST:

Flashing red light 
Bus 
Sign in a foreign language 
Dog 
Grocery store 
Railroad tracks 
Playground 
Billboard with child on it 
Billboard with cat on it 
Police officer

SUBURBS

School 
Flashing traffic light 
Grocery store 
Person on a bicycle 
Statue 
Church 
Playground 
Flower garden/snowman 
Children 
Pickup truck

RURAL LIST

Silo 
Railroad tracks 
Post office or school 
Cows 
Dirt road 
Pond or lake 
Tractor 
A fence 
Gas station 
Horses

TODDLER LIST

Tall building 
Lake or pond 
Person on a bicycle 
Bridge 
Red car 
Animal 
Truck 
Stop sign 
Store 
Person wearing a hat

Cow count 
First person to spot a field of cows gets to keep the cows in the field.  Count if you can if not guess at how many are in the field. Keep track of each persons cow count. First person to see a cemetery announces "cow loss" and chooses a person to eliminate their cows. That person goes back down to zero. First person to see a church doubles their cows. (Cows got married). First person to 500 cows wins!