little washing up liquid mixed with paint,then using a large paintbrush(decorating type)painting on foil,acetate or other slippery/shiny surfaces-children love the way the paint glides on the surface.

chalk on dark sugar paper

leaf or coin rubbings or anything else interesting with a raised surface.To make it more interesting tape a piece of paper over the items,children roll crayons on the side to reveal what's under the paper.