P.S.
This might be more for your 3 yo but this is what I brought for my kid on the plane or road since he was 2...
Kids books w/CD, where there is a sound like a ping when its time to turn the page.
Post in notes. My kid would stick them on everything and I love them b/c they come in all shapes, sizes and colors.
Popscicle sticks for playdoh. (throw a few in a baggie w/a playdoh so you don't have to worry about those darn cans and then you can just throw out after the trip)
A calculator.
Action songs CDs - like Laurie Berkner
Color Wonders
We always brought a small notebook for sticker time or coloring, just so we didn't have to worry about paper all over the place.
We bought a new cookie tray and used that for magnets which we put in a bag.
Mini flashlight