In preparation for a road trip, I was researching travel trays for my kids. I figured trays would help the kids keep busy coloring or playing with their small toys and simplify eating in the car. But everything I found was really expensive. My solution: cheap cookie sheets! My kids ride in high-back booster seats and it works great! There's a lip around the edge, so crayons or pencils won't roll off. And I can just slide them under the seat when they aren't in use!
You could pick up cookie sheets for next to nothing at garage sales or Goodwill. If you're looking for something a bit smaller or lighter, you could cut the bottom out of a lap-sized cardboard box, leaving just enough of a rim to keep everything from rolling.
Related: We've got lots more road trip tips!