This is a fun and quick idea for children (and adults!) of all ages.  You need:

A tray or dish

Shaving foam

Food colouring or liquid water colours*  (these give a brighter colour)

Lolly sticks

Paper

Squirt some shaving foam into the dish and smear around a bit.  Add some drops of your favourite colours.  Use the lolly stick (or fingers) to whizz the colours around in swirly patterns.  Place a sheet of paper on top and press down.  Remove the paper and use the side of the lolly stick to scrape off the excess foam.  Underneath you will find a beautiful and completely unique marbled pattern!

*You can by liquid water colours on amazon.  They are not cheap but a little goes a long way!