I know I've posted about this before, but I tried it again this morning with my preschool crowd and it was, yet again, a huge hit!
I poured blobs of paint onto a sheet of paper and they spread the paint by "driving" their cars through it. They loved looking at the different tracks that different types of wheels made.
After we finished painting, I filled the water table with soapy water and gave them each a sponge and they made a car wash! If you ask them, this was their favorite part!