There are a variety of ways to keep children entertained on roadtrips, ranging from games like "I-Spy" to the latest technological gadgets.
The Alphabet Games: There are a couple variations of this game. The first is to simply point out something that starts with the letter "A", followed by the letter "B" (I see (or I Spy) an Apple Orchard! I see a Boat!" Another variation is that the first person starts with the letter "A", but the next person starts with the last letter of the word that the first person used. For example, child 1 spots "Apple Orchard" and child 2 must then identify something that starts with the letter "D". Yet another variation is to pick a letter, and then pick a type of object. My family typically will do foods. For example, if the selected letter is "B" and you are using foods, each person thinks of foods that start with the letter, "B" such as Banana, Bread, Bagels, Beans, etc. If a person can't think of anything, then they are out for the remainder of the round until there is only one person left.
The Coloring Book (or Blank Paper): I always have a small bag with plain white paper, a coloring book, and crayons and/or washable markers. This is great for keeping pre-k - middle schoolers busy. You can also use the paper to play games like hangman or tic-tac-toe
Books - Bring a couple age-appropriate books per child. Try to get a new book that the child has not read before but would be interested in reading. Adults that are not driving can also read books to very young children who have not yet learned to read.
Make Funny Signs - my teens like to make funny signs (using the paper and markers) and hold them up for other travelers to see. They are usually along the lines of "I have to pee but they won't stop!" or "He passed gas and locked the windows!". the sign that gets the most laughing people in other vehicles wins.
Movies - watch a movie on a hand held device, laptop, or built in screen available in some vehicles.
License Plates - try to find as many plates from as many states as you can. Child who has the most different states wins (or don't make it a competition between kids and see how many the entire family as a whole can spot)
Take A Break - a break is good for both parents and kids. Stop at a rest stop, park, or restaurant with a play area and let the kids run around. It's good for parents and kids alike to stretch their legs and use the restrooms if available - and the kids will need to burn off some pent up energy. It doesn't need to be a long break - just 15-20 minutes or so!
Pillow & Blanket - if you are fortunate enough to have a child that tends to fall asleep on trips. make sure your child has a small pillow and blanket, and they might just sleep most of the trip. Being lost in dreamland is a pretty entertaining way to spend a roadtrip!
Cars - Matchbox or Hotwheels cars have accompanied myself, my siblings, and now our children on car trips.
Re-usable window decals - Get reusable window decals (like the kind used for holiday decorating) and let the kids "decorate" the car windows - just made sure that they are old enough to understand that they can not completely cover the window so the driver can see.
IPod or other music device and headphones: As a teenager, I often had a walkman or discman and headphones with me for long car rides. Today's version is the ipod, which the kids can also play games on (some games do not require wifi connection to run).
Travel Checkers & other travel games - there are a lot of magnetic travel games out there for kids, such as checkers. A travel sized Etch A Sketch and the Magnetic Doodle are also a good way to keep kids entertained.
Family Sing A Longs - These are probably a bit outdated, but crank up a song that everyone likes and let each family member belt out a part of the song if they want.
Video Games - There are a bunch of handheld video game options for young travelers. Just remember to pack spare batteries!
Navigator: Bring along a paper map and have your child mark on the map the route you are taking and the cities that you drive through. This is even better if you are driving through multiple states. If you stop anywhere for food or to see the sights, take pictures and tape to the map when you get home. The map can then be hung up at home to commemorate the trip!
Rock Paper Scissors - classic game and requires nothing but your hands!