When we moved from Pennsylvania to Indiana last year, Jon was 2.5 years old. I bought a few games for him, and kept them hidden until it was clear he was bored. Out came a game, and I repeated it the next hour or so. These games are great in the car, but they're also just plain fun to have inside your home, or if you need something to occupy your child in a waiting room.
Educo Colorblock Sea Turtle bead maze* -- This is a magnetic maze game. You use the wand (which can snap apart if you choose) to drag and drop a bead to the corresponding space in the turtle. I like that this maze has two shades of blue and two shades of green to help teach color variations.
The beads are behind a sheet of plexiglass. The recommended age is 3+ for small parts, and I'm guessing that caution is referring to the wand. So yeah, keep an eye on your kid. Overall, it's a durable, entertaining toy that should last for several children. It's quiet, too! The bumps of the road might frustrate your kid, though.
Similar to the turtle maze are some offerings from Melissa & Doug. One is color-based; the other is counting-based. I haven't tried either one so I can't say for sure if this would be fun in a moving vehicle. Might be too hard to keep the board perfectly flat and still.
We have this wooden Melissa & Doug maze puzzle that my mother-in-law picked up at Goodwill. You slide the vehicles around the maze track to where they should go (or where you want them to go). A fun puzzle and since the pieces are self-contained, it's great to have in the car.
I'd be remiss if I didn't mention the classic Etch-A-Sketch and a travel-size Magna-Doodle. And what about Wooly Willy or Hair-do Harriet? You'd have to use your discretion to make sure your child wouldn't rip it to shreds in your van. :)
What toys do you like to keep in your vehicle?