Simple to set up, but will be enjoyed by children of all ages.
Set aside an area of the garden, let the children make a car wash sign and stick it onto a cone or post. Cone a road way through the car wash for the children to ride their cars. Provide the children with sponges and buckets of warm water as well as towels to dry the car seat so that they can drive off.
For older children you may wish to use play money and a till so that they can pay before entering the car wash.