Sponge Blocks - I would cut them into a variety of shapes. These make great travel blocks because they are quiet, light, compact and have some texture to them to reduce sliding around in cars and other moving vehicles.
Will have to make these for the kids when we drive to Disney next year! 16 hours in a car with 2 kids under 13, this will help so much! DIY Lap Desk for Road Trips or on Airplane - Stuff to do for kids in the car