Nothing that makes noise or takes batteries.

Board books. Toddlers are murder on books with paper pages. Stick with classics like Goodnight Moon or Dr. Seuss, or if you want something more recent the Little Owl series by Divya Srinivasan, or Calm-Down Time by Elizabeth Verdick are really good.

Playground/beach toys, like a bucket / shovel / rake set that parents can keep in the car. They’ll likely get lost, so go for cheap and sturdy over premium and expensive.

Sidewalk chalk, like the huge fat ones.

The water based coloring books someone else mentioned are great, too. No cleanup (beyond the occasional water spill), they can be reused, and they’re great entertainment for the car.

And yeah, like others have said, coordinate with the parents.

And if you hate the parents, get a Furby knockoff or something else that makes noise.