A very simple idea that demonstrates the elements of art in a highly visible manner is to have a school "dress-up-day". This is not to be confused with a fancy dress as it involves no characters, masks, etc.
To begin with you need to discuss exactly what the elements of art are with the children (and show examples) - lines, shapes, colours, tones, patterns, textures. The children then take part in a "no uniform" day but must come to school dressed in clothes that show all these elements - hawaiian shirts, striped scarves, hats with feathers, polka dot skirts, odd brightly coloured socks, odd shoes with the laces replaced with coloured ribbon, fluffy jumpers, etc. etc.
Add another element to this by taking photos of the children and using these as a stimulus for drawing/painting.
Great also as a fund raising activity - each child must bring in a small amount of cash for a nominated charity.