The first we did was to cover the table with a big paper so that the kids can paint freely. Then we took a box with some wet tissues and poured some of the colors they wanted to have. In this case, white, yellow and orange and another box with green.
Kids would then drive their cars in the paint and later on onto the paper. Each car has different patterns on the wheels and that made a great effect on the big mural.
It was all in all a great activity that they want to repeat and which they talk about often!