I saw this shaving foam marbling idea ages ago and managed to forget but it was brought back to my attention on the artbar blog and I decided to try it.
You will need shaving foam, food colouring, a tray (or multiple trays if using more than one colour), a ruler or alternative straight edge, something to stir with, a bowl and paper.
Place a piece of paper face down on the foam and press gently so that all of the paper is touching the foam. Remove the paper and use the ruler to scrape off the foam into the bowl. Repeat with more paper adding more colouring when desired. Leave to dry and you should have a variety of marbled paper to play with the next day.