We are going on a trip also. My Son takes a therapy, Occupational. I asked for ideas from his therapist. She is so creative and gave me tons of things for him to do. The best idea I liked for Legos or anything with small pieces or toys with small pieces was using what you have at home! It is free! She said get a cake pan or a cookie sheet with some sides. If your child needs more room, get a longer pan, however with some sides so the pieces don't go falling on the floor so much. You could turn this over and use it as a tray to eat or to draw/color. you could put a paper towel or wipe it down with baby wipe or sanitizer also. You might not to use your best pan or maybe even get one just for the car trips. I could see it working for eating and coloring, doing small pieces like Legos or playing dolls with small parts/pieces,etc...
There is a toy shop where we bought my Son's tray table, however I don't like that it is so big. I usually use it in the house. It doesn't fold, however it does have legs and might work with your car seat. It has 2 side compartments where toys or other things can stay. We've used it some in the car. It does take up some room. It is called Designs for Children and you can also buy them at Mardel or Hobby Lobby or Michaels. They are plain, however at the Toy Store, they can personalize them for you and paint their name and a toy on there. It is plastic and they come in many different colors. It is something you can easily buy from the stores here in the metroplex.
You might also check resale shops or call them and ask what they have. Mudpuppy on Hulen is one, Kid to Kid in Fort Worth on Bryant Irvin is another. There are other places in Arlington and other cities that have consignment shops. They do have used supplies, so if you are okay with buying used, you could go that route.
You may be able to use a large tray that you have at home. We are into trays, and we have several metal, wood and painted trays. We've tried a few of them, however my Son likes to throw them on the floor. We'll pick something light just in case he aims it for our feet in the back seat! Most likely we will go with the cake pan or cookie sheet with some sides.
You might even try a Teacher's Supply Store, I know there is on one Hulen near Krogers and Mardel. Tbere are also fold down type trays made out of material with different characters on them. This worked when we had a car and one of our Foster Kids loved it! Now, we have a van and it doesn't work with the van, the van sits up too high and it doesn't even reach my Sons knees when placed on the back of the front seat. The car section of Wal-Mart or Target would be where you would find this. They have pockets and places for crayons, etc ... I highly recommend trying this out before your trip as it may not fit your vehicle, depending on what you have and how it works in your vehicle.
We've taken our Son one time to Fargo, ND and lots of times to Waco and to College Station. We have no family in the area and have to travel to visit family. We've had some experience, however we are still learning to get it down the best way!
Good Luck and let us know how it works out and if you thought of any other ideas with traveling with your child!