You will need:
Oven Tray
Paper
Marbles - any size
Coloured Paints
Place paper into oven tray, try to choose paper size as close to tray size as possible. I cut an A4 sheet to fit.
Pop your marbles in and off you go, roll away from side to side, top to bottom.
Make patterns and roll the marbles through as many blobs of paint as you can.