We enjoyed watching how the colours from the old felt tip pens used in this craft spread, and the resultant picture is very pretty. Make sure you use a good thick paper though, or the result might be disappointing.
You will need:
Old felt tips in reds, yellows, oranges and greens (this is a good craft to use up the last bits of ink)
Paint a cloud shape (which will be the "leaves" of your tree) with water onto the top half of your paper.
Sam painting a cloud shape onto the paper
Colour in the cloud with the old pens. The inks will run together.
Colouring onto the cloud with felt tips - the colours are starting to run together
When the cloud shape is completely dry, use the brown pen to draw on a trunk and branches.