Make up some different coloured runny paints and put a couple of drops of paint onto the paper. Then, use a straw to blow the paint in different directions. Put a different colour of paint onto the paper next and do the same again to make 'fireworks' going in different directions. You can use either black or white paper depending on what you have around.
This is one for outside or a large, easily cleanable space. Try loading a paint brush with paint and using a flicking motion to "splat" the paint on to the paper. Add more "splats" until you have made your own Jackson Pollock style masterpiece.